diff --git a/includes/AutoLoader.php b/includes/AutoLoader.php
index b893bc9e147..abbc62c7f6c 100644
--- a/includes/AutoLoader.php
+++ b/includes/AutoLoader.php
@@ -134,6 +134,7 @@ class AutoLoader {
 			'MediaWiki\\Edit\\' => __DIR__ . '/edit/',
 			'MediaWiki\\EditPage\\' => __DIR__ . '/editpage/',
 			'MediaWiki\\Linker\\' => __DIR__ . '/linker/',
+			'MediaWiki\\Message\\' => __DIR__ . '/Message',
 			'MediaWiki\\Permissions\\' => __DIR__ . '/Permissions/',
 			'MediaWiki\\Preferences\\' => __DIR__ . '/preferences/',
 			'MediaWiki\\Rest\\' => __DIR__ . '/Rest/',
@@ -143,6 +144,7 @@ class AutoLoader {
 			'MediaWiki\\Sparql\\' => __DIR__ . '/sparql/',
 			'MediaWiki\\Storage\\' => __DIR__ . '/Storage/',
 			'MediaWiki\\Tidy\\' => __DIR__ . '/tidy/',
+			'Wikimedia\\Message\\' => __DIR__ . '/libs/Message/',
 			'Wikimedia\\ParamValidator\\' => __DIR__ . '/libs/ParamValidator/',
 			'Wikimedia\\Services\\' => __DIR__ . '/libs/services/',
 		];
diff --git a/includes/MediaWikiServices.php b/includes/MediaWikiServices.php
index 6013aafc9ea..c89fa4a23f0 100644
--- a/includes/MediaWikiServices.php
+++ b/includes/MediaWikiServices.php
@@ -18,6 +18,7 @@ use MediaWiki\Block\BlockManager;
 use MediaWiki\Block\BlockRestrictionStore;
 use MediaWiki\FileBackend\FSFile\TempFSFileFactory;
 use MediaWiki\Http\HttpRequestFactory;
+use Wikimedia\Message\IMessageFormatterFactory;
 use MediaWiki\Page\MovePageFactory;
 use MediaWiki\Permissions\PermissionManager;
 use MediaWiki\Preferences\PreferencesFactory;
@@ -709,6 +710,14 @@ class MediaWikiServices extends ServiceContainer {
 		return $this->getService( 'MessageCache' );
 	}
 
+	/**
+	 * @since 1.34
+	 * @return IMessageFormatterFactory
+	 */
+	public function getMessageFormatterFactory() {
+		return $this->getService( 'MessageFormatterFactory' );
+	}
+
 	/**
 	 * @since 1.28
 	 * @return MimeAnalyzer
diff --git a/includes/Message/MessageFormatterFactory.php b/includes/Message/MessageFormatterFactory.php
new file mode 100644
index 00000000000..101224a6a82
--- /dev/null
+++ b/includes/Message/MessageFormatterFactory.php
@@ -0,0 +1,29 @@
+<?php
+
+namespace MediaWiki\Message;
+
+use Wikimedia\Message\IMessageFormatterFactory;
+use Wikimedia\Message\ITextFormatter;
+
+/**
+ * The MediaWiki-specific implementation of IMessageFormatterFactory
+ */
+class MessageFormatterFactory implements IMessageFormatterFactory {
+	private $textFormatters = [];
+
+	/**
+	 * Required parameters may be added to this function without deprecation.
+	 * External callers should use MediaWikiServices::getMessageFormatterFactory().
+	 *
+	 * @internal
+	 */
+	public function __construct() {
+	}
+
+	public function getTextFormatter( $langCode ): ITextFormatter {
+		if ( !isset( $this->textFormatters[$langCode] ) ) {
+			$this->textFormatters[$langCode] = new TextFormatter( $langCode );
+		}
+		return $this->textFormatters[$langCode];
+	}
+}
diff --git a/includes/Message/TextFormatter.php b/includes/Message/TextFormatter.php
new file mode 100644
index 00000000000..f5eeb16f247
--- /dev/null
+++ b/includes/Message/TextFormatter.php
@@ -0,0 +1,74 @@
+<?php
+
+namespace MediaWiki\Message;
+
+use Wikimedia\Message\ITextFormatter;
+use Wikimedia\Message\ListParam;
+use Wikimedia\Message\MessageParam;
+use Wikimedia\Message\MessageValue;
+use Wikimedia\Message\ParamType;
+use Message;
+
+/**
+ * The MediaWiki-specific implementation of ITextFormatter
+ */
+class TextFormatter implements ITextFormatter {
+	/** @var string */
+	private $langCode;
+
+	/**
+	 * Construct a TextFormatter.
+	 *
+	 * The type signature may change without notice as dependencies are added
+	 * to the constructor. External callers should use
+	 * MediaWikiServices::getMessageFormatterFactory()
+	 *
+	 * @internal
+	 */
+	public function __construct( $langCode ) {
+		$this->langCode = $langCode;
+	}
+
+	/**
+	 * Allow the Message class to be mocked in tests by constructing objects in
+	 * a protected method.
+	 *
+	 * @internal
+	 * @param string $key
+	 * @return Message
+	 */
+	protected function createMessage( $key ) {
+		return new Message( $key );
+	}
+
+	public function getLangCode() {
+		return $this->langCode;
+	}
+
+	private static function convertParam( MessageParam $param ) {
+		if ( $param instanceof ListParam ) {
+			$convertedElements = [];
+			foreach ( $param->getValue() as $element ) {
+				$convertedElements[] = self::convertParam( $element );
+			}
+			return Message::listParam( $convertedElements, $param->getListType() );
+		} elseif ( $param instanceof MessageParam ) {
+			if ( $param->getType() === ParamType::TEXT ) {
+				return $param->getValue();
+			} else {
+				return [ $param->getType() => $param->getValue() ];
+			}
+		} else {
+			throw new \InvalidArgumentException( 'Invalid message parameter type' );
+		}
+	}
+
+	public function format( MessageValue $mv ) {
+		$message = $this->createMessage( $mv->getKey() );
+		foreach ( $mv->getParams() as $param ) {
+			$message->params( self::convertParam( $param ) );
+		}
+		$message->inLanguage( $this->langCode );
+		return $message->text();
+	}
+}
diff --git a/includes/Rest/LocalizedHttpException.php b/includes/Rest/LocalizedHttpException.php
new file mode 100644
index 00000000000..10d3a4034a6
--- /dev/null
+++ b/includes/Rest/LocalizedHttpException.php
@@ -0,0 +1,11 @@
+<?php
+
+namespace MediaWiki\Rest;
+
+use Wikimedia\Message\MessageValue;
+
+class LocalizedHttpException extends HttpException {
+	public function __construct( MessageValue $message, $code = 500 ) {
+		parent::__construct( 'Localized exception with key ' . $message->getKey(), $code );
+	}
+}
diff --git a/includes/ServiceWiring.php b/includes/ServiceWiring.php
index b30726415e1..7000bd3b79d 100644
--- a/includes/ServiceWiring.php
+++ b/includes/ServiceWiring.php
@@ -52,6 +52,8 @@ use MediaWiki\Linker\LinkRenderer;
 use MediaWiki\Linker\LinkRendererFactory;
 use MediaWiki\Logger\LoggerFactory;
 use MediaWiki\MediaWikiServices;
+use Wikimedia\Message\IMessageFormatterFactory;
+use MediaWiki\Message\MessageFormatterFactory;
 use MediaWiki\Page\MovePageFactory;
 use MediaWiki\Permissions\PermissionManager;
 use MediaWiki\Preferences\PreferencesFactory;
@@ -350,6 +352,11 @@ return [
 		);
 	},
 
+	'MessageFormatterFactory' =>
+	function ( MediaWikiServices $services ) : IMessageFormatterFactory {
+		return new MessageFormatterFactory();
+	},
+
 	'MimeAnalyzer' => function ( MediaWikiServices $services ) : MimeAnalyzer {
 		$logger = LoggerFactory::getInstance( 'Mime' );
 		$mainConfig = $services->getMainConfig();
diff --git a/includes/libs/Message/IMessageFormatterFactory.php b/includes/libs/Message/IMessageFormatterFactory.php
new file mode 100644
index 00000000000..337ea82436d
--- /dev/null
+++ b/includes/libs/Message/IMessageFormatterFactory.php
@@ -0,0 +1,18 @@
+<?php
+
+namespace Wikimedia\Message;
+
+/**
+ * A simple factory providing a message formatter for a given language code.
+ *
+ * @see ITextFormatter
+ */
+interface IMessageFormatterFactory {
+	/**
+	 * Get a text message formatter for a given language.
+	 *
+	 * @param string $langCode The language code
+	 * @return ITextFormatter
+	 */
+	public function getTextFormatter( $langCode ): ITextFormatter;
+}
diff --git a/includes/libs/Message/ITextFormatter.php b/includes/libs/Message/ITextFormatter.php
new file mode 100644
index 00000000000..00f6e990076
--- /dev/null
+++ b/includes/libs/Message/ITextFormatter.php
@@ -0,0 +1,43 @@
+<?php
+
+namespace Wikimedia\Message;
+
+/**
+ * ITextFormatter is a simplified interface to the Message class. It converts
+ * MessageValue message specifiers to localized text in a certain language.
+ *
+ * MessageValue supports message keys, and parameters with a wide variety of
+ * types. It does not expose any details of how messages are retrieved from
+ * storage or what format they are stored in.
+ *
+ * Thus, TextFormatter supports single message keys, but not the concept of
+ * presence or absence of a key from storage. So it does not support
+ * fallback sequences of multiple keys.
+ *
+ * The caller cannot modify the details of message translation, such as which
+ * of multiple sources the message is taken from. Any such flags may be injected
+ * into the factory constructor.
+ *
+ * Implementations of TextFormatter are not required to perfectly format
+ * any message in any language. Implementations should make a best effort to
+ * produce human-readable text.
+ *
+ * @package MediaWiki\MessageFormatter
+ */
+interface ITextFormatter {
+	/**
+	 * Get the internal language code in which format() is
+	 * @return string
+	 */
+	function getLangCode();
+
+	/**
+	 * Convert a MessageValue to text.
+	 *
+	 * The result is not safe for use as raw HTML.
+	 *
+	 * @param MessageValue $message
+	 * @return string
+	 */
+	function format( MessageValue $message );
+}
diff --git a/includes/libs/Message/ListParam.php b/includes/libs/Message/ListParam.php
new file mode 100644
index 00000000000..c6a9c65d577
--- /dev/null
+++ b/includes/libs/Message/ListParam.php
@@ -0,0 +1,52 @@
+<?php
+
+namespace Wikimedia\Message;
+
+/**
+ * The class for list parameters
+ */
+class ListParam extends MessageParam {
+	private $listType;
+
+	/**
+	 * @param string $listType One of the ListType constants:
+	 *   - ListType::COMMA: A comma-separated list
+	 *   - ListType::SEMICOLON: A semicolon-separated list
+	 *   - ListType::PIPE: A pipe-separated list
+	 *   - ListType::TEXT: A natural language list, separated by commas and
+	 *     the word "and".
+	 * @param (MessageParam|string)[] $elements An array of parameters
+	 */
+	public function __construct( $listType, array $elements ) {
+		$this->type = ParamType::LIST;
+		$this->listType = $listType;
+		$this->value = [];
+		foreach ( $elements as $element ) {
+			if ( $element instanceof MessageParam ) {
+				$this->value[] = $element;
+			} elseif ( is_scalar( $element ) ) {
+				$this->value[] = new TextParam( ParamType::TEXT, $element );
+			} else {
+				throw new \InvalidArgumentException(
+					'ListParam elements must be MessageParam or scalar' );
+			}
+		}
+	}
+
+	/**
+	 * Get the type of the list
+	 *
+	 * @return string One of the ListType constants
+	 */
+	public function getListType() {
+		return $this->listType;
+	}
+
+	public function dump() {
+		$contents = '';
+		foreach ( $this->value as $element ) {
+			$contents .= $element->dump();
+		}
+		return "<{$this->type} listType=\"{$this->listType}\">$contents</{$this->type}>";
+	}
+}
diff --git a/includes/libs/Message/ListType.php b/includes/libs/Message/ListType.php
new file mode 100644
index 00000000000..60f3a822332
--- /dev/null
+++ b/includes/libs/Message/ListType.php
@@ -0,0 +1,22 @@
+<?php
+
+namespace Wikimedia\Message;
+
+/**
+ * The constants used to specify list types. The values of the constants are an
+ * unstable implementation detail and correspond to the names of the list types
+ * in the Message class.
+ */
+class ListType {
+	/** A comma-separated list */
+	const COMMA = 'comma';
+
+	/** A semicolon-separated list */
+	const SEMICOLON = 'semicolon';
+
+	/** A pipe-separated list */
+	const PIPE = 'pipe';
+
+	/** A natural-language list separated by "and" */
+	const AND = 'text';
+}
diff --git a/includes/libs/Message/MessageParam.php b/includes/libs/Message/MessageParam.php
new file mode 100644
index 00000000000..8162212c068
--- /dev/null
+++ b/includes/libs/Message/MessageParam.php
@@ -0,0 +1,36 @@
+<?php
+
+namespace Wikimedia\Message;
+
+/**
+ * The base class for message parameters.
+ */
+abstract class MessageParam {
+	protected $type;
+	protected $value;
+
+	/**
+	 * Get the type of the parameter.
+	 *
+	 * @return string One of the ParamType constants
+	 */
+	public function getType() {
+		return $this->type;
+	}
+
+	/**
+	 * Get the input value of the parameter
+	 *
+	 * @return int|float|string|array
+	 */
+	public function getValue() {
+		return $this->value;
+	}
+
+	/**
+	 * Dump the object for testing/debugging
+	 *
+	 * @return string
+	 */
+	abstract public function dump();
+}
diff --git a/includes/libs/Message/MessageValue.php b/includes/libs/Message/MessageValue.php
new file mode 100644
index 00000000000..13b97f224ae
--- /dev/null
+++ b/includes/libs/Message/MessageValue.php
@@ -0,0 +1,258 @@
+<?php
+
+namespace Wikimedia\Message;
+
+/**
+ * A MessageValue holds a key and an array of parameters
+ */
+class MessageValue {
+	/** @var string */
+	private $key;
+
+	/** @var MessageParam[] */
+	private $params;
+
+	/**
+	 * @param string $key
+	 * @param array $params Each element of the parameter array
+	 *   may be either a MessageParam or a scalar. If it is a scalar, it is
+	 *   converted to a parameter of type TEXT.
+	 */
+	public function __construct( $key, $params = [] ) {
+		$this->key = $key;
+		$this->params = [];
+		$this->params( ...$params );
+	}
+
+	/**
+	 * Get the message key
+	 *
+	 * @return string
+	 */
+	public function getKey() {
+		return $this->key;
+	}
+
+	/**
+	 * Get the parameter array
+	 *
+	 * @return MessageParam[]
+	 */
+	public function getParams() {
+		return $this->params;
+	}
+
+	/**
+	 * Chainable mutator which adds text parameters and MessageParam parameters
+	 *
+	 * @param mixed ...$values Scalar or MessageParam values
+	 * @return MessageValue
+	 */
+	public function params( ...$values ) {
+		foreach ( $values as $value ) {
+			if ( $value instanceof MessageParam ) {
+				$this->params[] = $value;
+			} else {
+				$this->params[] = new TextParam( ParamType::TEXT, $value );
+			}
+		}
+		return $this;
+	}
+
+	/**
+	 * Chainable mutator which adds text parameters with a common type
+	 *
+	 * @param string $type One of the ParamType constants
+	 * @param mixed ...$values Scalar values
+	 * @return MessageValue
+	 */
+	public function textParamsOfType( $type, ...$values ) {
+		foreach ( $values as $value ) {
+			$this->params[] = new TextParam( $type, $value );
+		}
+		return $this;
+	}
+
+	/**
+	 * Chainable mutator which adds list parameters with a common type
+	 *
+	 * @param string $listType One of the ListType constants
+	 * @param array ...$values Each value should be an array of list items.
+	 * @return MessageValue
+	 */
+	public function listParamsOfType( $listType, ...$values ) {
+		foreach ( $values as $value ) {
+			$this->params[] = new ListParam( $listType, $value );
+		}
+		return $this;
+	}
+
+	/**
+	 * Chainable mutator which adds parameters of type text.
+	 *
+	 * @param string ...$values
+	 * @return MessageValue
+	 */
+	public function textParams( ...$values ) {
+		return $this->textParamsOfType( ParamType::TEXT, ...$values );
+	}
+
+	/**
+	 * Chainable mutator which adds numeric parameters
+	 *
+	 * @param mixed ...$values
+	 * @return MessageValue
+	 */
+	public function numParams( ...$values ) {
+		return $this->textParamsOfType( ParamType::NUM, ...$values );
+	}
+
+	/**
+	 * Chainable mutator which adds parameters which are a duration specified
+	 * in seconds. This is similar to timePeriodParams() except that the result
+	 * will be more verbose.
+	 *
+	 * @param int|float ...$values
+	 * @return MessageValue
+	 */
+	public function longDurationParams( ...$values ) {
+		return $this->textParamsOfType( ParamType::DURATION_LONG, ...$values );
+	}
+
+	/**
+	 * Chainable mutator which adds parameters which are a time period in seconds.
+	 * This is similar to durationParams() except that the result will be more
+	 * compact.
+	 *
+	 * @param int|float ...$values
+	 * @return MessageValue
+	 */
+	public function shortDurationParams( ...$values ) {
+		return $this->textParamsOfType( ParamType::DURATION_SHORT, ...$values );
+	}
+
+	/**
+	 * Chainable mutator which adds parameters which are an expiry timestamp
+	 * as used in the MediaWiki database schema.
+	 *
+	 * @param string ...$values
+	 * @return MessageValue
+	 */
+	public function expiryParams( ...$values ) {
+		return $this->textParamsOfType( ParamType::EXPIRY, ...$values );
+	}
+
+	/**
+	 * Chainable mutator which adds parameters which are a number of bytes.
+	 *
+	 * @param int ...$values
+	 * @return MessageValue
+	 */
+	public function sizeParams( ...$values ) {
+		return $this->textParamsOfType( ParamType::SIZE, ...$values );
+	}
+
+	/**
+	 * Chainable mutator which adds parameters which are a number of bits per
+	 * second.
+	 *
+	 * @param int|float ...$values
+	 * @return MessageValue
+	 */
+	public function bitrateParams( ...$values ) {
+		return $this->textParamsOfType( ParamType::BITRATE, ...$values );
+	}
+
+	/**
+	 * Chainable mutator which adds parameters of type "raw".
+	 *
+	 * @param mixed ...$values
+	 * @return MessageValue
+	 */
+	public function rawParams( ...$values ) {
+		return $this->textParamsOfType( ParamType::RAW, ...$values );
+	}
+
+	/**
+	 * Chainable mutator which adds parameters of type "plaintext".
+	 */
+	public function plaintextParams( ...$values ) {
+		return $this->textParamsOfType( ParamType::PLAINTEXT, ...$values );
+	}
+
+	/**
+	 * Chainable mutator which adds comma lists. Each comma list is an array of
+	 * list elements, and each list element is either a MessageParam or a
+	 * string. String parameters are converted to parameters of type "text".
+	 *
+	 * The list parameters thus created are formatted as a comma-separated list,
+	 * or some local equivalent.
+	 *
+	 * @param (MessageParam|string)[] ...$values
+	 * @return MessageValue
+	 */
+	public function commaListParams( ...$values ) {
+		return $this->listParamsOfType( ListType::COMMA, ...$values );
+	}
+
+	/**
+	 * Chainable mutator which adds semicolon lists. Each semicolon list is an
+	 * array of list elements, and each list element is either a MessageParam
+	 * or a string. String parameters are converted to parameters of type
+	 * "text".
+	 *
+	 * The list parameters thus created are formatted as a semicolon-separated
+	 * list, or some local equivalent.
+	 *
+	 * @param (MessageParam|string)[] ...$values
+	 * @return MessageValue
+	 */
+	public function semicolonListParams( ...$values ) {
+		return $this->listParamsOfType( ListType::SEMICOLON, ...$values );
+	}
+
+	/**
+	 * Chainable mutator which adds pipe lists. Each pipe list is an array of
+	 * list elements, and each list element is either a MessageParam or a
+	 * string. String parameters are converted to parameters of type "text".
+	 *
+	 * The list parameters thus created are formatted as a pipe ("|") -separated
+	 * list, or some local equivalent.
+	 *
+	 * @param (MessageParam|string)[] ...$values
+	 * @return MessageValue
+	 */
+	public function pipeListParams( ...$values ) {
+		return $this->listParamsOfType( ListType::PIPE, ...$values );
+	}
+
+	/**
+	 * Chainable mutator which adds text lists. Each text list is an array of
+	 * list elements, and each list element is either a MessageParam or a
+	 * string. String parameters are converted to parameters of type "text".
+	 *
+	 * The list parameters thus created, when formatted, are joined as in natural
+	 * language. In English, this means a comma-separated list, with the last
+	 * two elements joined with "and".
+	 *
+	 * @param (MessageParam|string)[] ...$values
+	 * @return MessageValue
+	 */
+	public function textListParams( ...$values ) {
+		return $this->listParamsOfType( ListType::AND, ...$values );
+	}
+
+	/**
+	 * Dump the object for testing/debugging
+	 *
+	 * @return string
+	 */
+	public function dump() {
+		$contents = '';
+		foreach ( $this->params as $param ) {
+			$contents .= $param->dump();
+		}
+		return '<message key="' . htmlspecialchars( $this->key ) . '">' .
+			$contents . '</message>';
+	}
+}
diff --git a/includes/libs/Message/ParamType.php b/includes/libs/Message/ParamType.php
new file mode 100644
index 00000000000..890ef38ee41
--- /dev/null
+++ b/includes/libs/Message/ParamType.php
@@ -0,0 +1,47 @@
+<?php
+
+namespace Wikimedia\Message;
+
+/**
+ * The constants used to specify parameter types. The values of the constants
+ * are an unstable implementation detail, and correspond to the names of the
+ * parameter types in the Message class.
+ */
+class ParamType {
+	/** A simple text parameter */
+	const TEXT = 'text';
+
+	/** A number, to be formatted using local digits and separators */
+	const NUM = 'num';
+
+	/** A number of seconds, to be formatted as natural language text. */
+	const DURATION_LONG = 'duration';
+
+	/** A number of seconds, to be formatted in an abbreviated way. */
+	const DURATION_SHORT = 'timeperiod';
+
+	/**
+	 * An expiry time for a block. The input is either a timestamp in one
+	 * of the formats accepted by the Wikimedia\Timestamp library, or
+	 * "infinity" for an infinite block.
+	 */
+	const EXPIRY = 'expiry';
+
+	/** A number of bytes. */
+	const SIZE = 'size';
+
+	/** A number of bits per second. */
+	const BITRATE = 'bitrate';
+
+	/** The list type (ListParam) */
+	const LIST = 'list';
+
+	/**
+	 * A text parameter which is substituted after preprocessing, and so is
+	 * not available to the preprocessor and cannot be modified by it.
+	 */
+	const RAW = 'raw';
+
+	/** Reserved for future use. */
+	const PLAINTEXT = 'plaintext';
+}
diff --git a/includes/libs/Message/TextParam.php b/includes/libs/Message/TextParam.php
new file mode 100644
index 00000000000..c1a1f089ee8
--- /dev/null
+++ b/includes/libs/Message/TextParam.php
@@ -0,0 +1,37 @@
+<?php
+
+namespace Wikimedia\Message;
+
+class TextParam extends MessageParam {
+	/**
+	 * Construct a text parameter
+	 *
+	 * @param string $type May be one of:
+	 *   - ParamType::TEXT: A simple text parameter
+	 *   - ParamType::NUM: A number, to be formatted using local digits and
+	 *     separators
+	 *   - ParamType::DURATION_LONG: A number of seconds, to be formatted as natural
+	 *     language text.
+	 *   - ParamType::DURATION_SHORT: A number of seconds, to be formatted in an
+	 *     abbreviated way.
+	 *   - ParamType::EXPIRY: An expiry time for a block. The input is either
+	 *     a timestamp in one of the formats accepted by the Wikimedia\Timestamp
+	 *     library, or "infinity" for an infinite block.
+	 *   - ParamType::SIZE: A number of bytes.
+	 *   - ParamType::BITRATE: A number of bits per second.
+	 *   - ParamType::RAW: A text parameter which is substituted after
+	 *     preprocessing, and so is not available to the preprocessor and cannot
+	 *     be modified by it.
+	 *   - ParamType::PLAINTEXT: Reserved for future use.
+	 *
+	 * @param string|int|float $value
+	 */
+	public function __construct( $type, $value ) {
+		$this->type = $type;
+		$this->value = $value;
+	}
+
+	public function dump() {
+		return "<{$this->type}>" . htmlspecialchars( $this->value ) . "</{$this->type}>";
+	}
+}
diff --git a/tests/phpunit/includes/Message/TextFormatterTest.php b/tests/phpunit/includes/Message/TextFormatterTest.php
new file mode 100644
index 00000000000..233810fe1b5
--- /dev/null
+++ b/tests/phpunit/includes/Message/TextFormatterTest.php
@@ -0,0 +1,59 @@
+<?php
+
+namespace MediaWiki\Tests\Message;
+
+use MediaWiki\Message\TextFormatter;
+use MediaWikiTestCase;
+use Message;
+use Wikimedia\Message\MessageValue;
+use Wikimedia\Message\ParamType;
+use Wikimedia\Message\TextParam;
+
+/**
+ * @covers \MediaWiki\Message\TextFormatter
+ * @covers \Wikimedia\Message\MessageValue
+ * @covers \Wikimedia\Message\ListParam
+ * @covers \Wikimedia\Message\TextParam
+ * @covers \Wikimedia\Message\MessageParam
+ */
+class TextFormatterTest extends MediaWikiTestCase {
+	private function createTextFormatter( $langCode ) {
+		return new class( $langCode ) extends TextFormatter {
+			public function __construct( $langCode ) {
+				parent::__construct( $langCode );
+			}
+
+			protected function createMessage( $key ) {
+				return new FakeMessage( $key );
+			}
+		};
+	}
+
+	public function testGetLangCode() {
+		$formatter = $this->createTextFormatter( 'fr' );
+		$this->assertSame( 'fr', $formatter->getLangCode() );
+	}
+
+	public function testFormatBitrate() {
+		$formatter = $this->createTextFormatter( 'en' );
+		$mv = ( new MessageValue( 'test' ) )->bitrateParams( 100, 200 );
+		$result = $formatter->format( $mv );
+		$this->assertSame( 'test 100 bps 200 bps', $result );
+	}
+
+	public function testFormatList() {
+		$formatter = $this->createTextFormatter( 'en' );
+		$mv = ( new MessageValue( 'test' ) )->commaListParams( [
+			'a',
+			new TextParam( ParamType::BITRATE, 100 ),
+		] );
+		$result = $formatter->format( $mv );
+		$this->assertSame( 'test a, 100 bps $2', $result );
+	}
+}
+
+class FakeMessage extends Message {
+	public function fetchMessage() {
+		return "{$this->getKey()} $1 $2";
+	}
+}
diff --git a/tests/phpunit/includes/libs/Message/MessageValueTest.php b/tests/phpunit/includes/libs/Message/MessageValueTest.php
new file mode 100644
index 00000000000..04dfa4e7e99
--- /dev/null
+++ b/tests/phpunit/includes/libs/Message/MessageValueTest.php
@@ -0,0 +1,219 @@
+<?php
+
+namespace Wikimedia\Tests\Message;
+
+use Wikimedia\Message\ListType;
+use Wikimedia\Message\MessageValue;
+use Wikimedia\Message\ParamType;
+use Wikimedia\Message\TextParam;
+use MediaWikiTestCase;
+
+/**
+ * @covers \Wikimedia\Message\MessageValue
+ * @covers \Wikimedia\Message\ListParam
+ * @covers \Wikimedia\Message\TextParam
+ * @covers \Wikimedia\Message\MessageParam
+ */
+class MessageValueTest extends MediaWikiTestCase {
+	public static function provideConstruct() {
+		return [
+			[
+				[],
+				'<message key="key"></message>',
+			],
+			[
+				[ 'a' ],
+				'<message key="key"><text>a</text></message>'
+			],
+			[
+				[ new TextParam( ParamType::BITRATE, 100 ) ],
+				'<message key="key"><bitrate>100</bitrate></message>'
+			],
+		];
+	}
+
+	/** @dataProvider provideConstruct */
+	public function testConstruct( $input, $expected ) {
+		$mv = new MessageValue( 'key', $input );
+		$this->assertSame( $expected, $mv->dump() );
+	}
+
+	public function testGetKey() {
+		$mv = new MessageValue( 'key' );
+		$this->assertSame( 'key', $mv->getKey() );
+	}
+
+	public function testParams() {
+		$mv = new MessageValue( 'key' );
+		$mv->params( 1, 'x' );
+		$mv2 = $mv->params( new TextParam( ParamType::BITRATE, 100 ) );
+		$this->assertSame(
+			'<message key="key"><text>1</text><text>x</text><bitrate>100</bitrate></message>',
+			$mv->dump() );
+		$this->assertSame( $mv, $mv2 );
+	}
+
+	public function testTextParamsOfType() {
+		$mv = new MessageValue( 'key' );
+		$mv2 = $mv->textParamsOfType( ParamType::BITRATE, 1, 2 );
+		$this->assertSame( '<message key="key">' .
+			'<bitrate>1</bitrate><bitrate>2</bitrate>' .
+			'</message>',
+			$mv->dump() );
+		$this->assertSame( $mv, $mv2 );
+	}
+
+	public function testListParamsOfType() {
+		$mv = new MessageValue( 'key' );
+		$mv2 = $mv->listParamsOfType( ListType::COMMA, [ 'a' ], [ 'b', 'c' ] );
+		$this->assertSame( '<message key="key">' .
+			'<list listType="comma"><text>a</text></list>' .
+			'<list listType="comma"><text>b</text><text>c</text></list>' .
+			'</message>',
+			$mv->dump() );
+		$this->assertSame( $mv, $mv2 );
+	}
+
+	public function testTextParams() {
+		$mv = new MessageValue( 'key' );
+		$mv2 = $mv->textParams( 'a', 'b' );
+		$this->assertSame( '<message key="key">' .
+			'<text>a</text>' .
+			'<text>b</text>' .
+			'</message>',
+			$mv->dump() );
+		$this->assertSame( $mv, $mv2 );
+	}
+
+	public function testNumParams() {
+		$mv = new MessageValue( 'key' );
+		$mv2 = $mv->numParams( 1, 2 );
+		$this->assertSame( '<message key="key">' .
+			'<num>1</num>' .
+			'<num>2</num>' .
+			'</message>',
+			$mv->dump() );
+		$this->assertSame( $mv, $mv2 );
+	}
+
+	public function testLongDurationParams() {
+		$mv = new MessageValue( 'key' );
+		$mv2 = $mv->longDurationParams( 1, 2 );
+		$this->assertSame( '<message key="key">' .
+			'<duration>1</duration>' .
+			'<duration>2</duration>' .
+			'</message>',
+			$mv->dump() );
+		$this->assertSame( $mv, $mv2 );
+	}
+
+	public function testShortDurationParams() {
+		$mv = new MessageValue( 'key' );
+		$mv2 = $mv->shortDurationParams( 1, 2 );
+		$this->assertSame( '<message key="key">' .
+			'<timeperiod>1</timeperiod>' .
+			'<timeperiod>2</timeperiod>' .
+			'</message>',
+			$mv->dump() );
+		$this->assertSame( $mv, $mv2 );
+	}
+
+	public function testExpiryParams() {
+		$mv = new MessageValue( 'key' );
+		$mv2 = $mv->expiryParams( 1, 2 );
+		$this->assertSame( '<message key="key">' .
+			'<expiry>1</expiry>' .
+			'<expiry>2</expiry>' .
+			'</message>',
+			$mv->dump() );
+		$this->assertSame( $mv, $mv2 );
+	}
+
+	public function testSizeParams() {
+		$mv = new MessageValue( 'key' );
+		$mv2 = $mv->sizeParams( 1, 2 );
+		$this->assertSame( '<message key="key">' .
+			'<size>1</size>' .
+			'<size>2</size>' .
+			'</message>',
+			$mv->dump() );
+		$this->assertSame( $mv, $mv2 );
+	}
+
+	public function testBitrateParams() {
+		$mv = new MessageValue( 'key' );
+		$mv2 = $mv->bitrateParams( 1, 2 );
+		$this->assertSame( '<message key="key">' .
+			'<bitrate>1</bitrate>' .
+			'<bitrate>2</bitrate>' .
+			'</message>',
+			$mv->dump() );
+		$this->assertSame( $mv, $mv2 );
+	}
+
+	public function testRawParams() {
+		$mv = new MessageValue( 'key' );
+		$mv2 = $mv->rawParams( 1, 2 );
+		$this->assertSame( '<message key="key">' .
+			'<raw>1</raw>' .
+			'<raw>2</raw>' .
+			'</message>',
+			$mv->dump() );
+		$this->assertSame( $mv, $mv2 );
+	}
+
+	public function testPlaintextParams() {
+		$mv = new MessageValue( 'key' );
+		$mv2 = $mv->plaintextParams( 1, 2 );
+		$this->assertSame( '<message key="key">' .
+			'<plaintext>1</plaintext>' .
+			'<plaintext>2</plaintext>' .
+			'</message>',
+			$mv->dump() );
+		$this->assertSame( $mv, $mv2 );
+	}
+
+	public function testCommaListParams() {
+		$mv = new MessageValue( 'key' );
+		$mv2 = $mv->commaListParams( [ 'a', 'b' ] );
+		$this->assertSame( '<message key="key">' .
+			'<list listType="comma">' .
+			'<text>a</text><text>b</text>' .
+			'</list></message>',
+			$mv->dump() );
+		$this->assertSame( $mv, $mv2 );
+	}
+
+	public function tesSemicolonListParams() {
+		$mv = new MessageValue( 'key' );
+		$mv2 = $mv->semicolonListParams( [ 'a', 'b' ] );
+		$this->assertSame( '<message key="key">' .
+			'<list listType="semicolon">' .
+			'<text>a</text><text>b</text>' .
+			'</list></message>',
+			$mv->dump() );
+		$this->assertSame( $mv, $mv2 );
+	}
+
+	public function testPipeListParams() {
+		$mv = new MessageValue( 'key' );
+		$mv2 = $mv->pipeListParams( [ 'a', 'b' ] );
+		$this->assertSame( '<message key="key">' .
+			'<list listType="pipe">' .
+			'<text>a</text><text>b</text>' .
+			'</list></message>',
+			$mv->dump() );
+		$this->assertSame( $mv, $mv2 );
+	}
+
+	public function testTextListParams() {
+		$mv = new MessageValue( 'key' );
+		$mv2 = $mv->textListParams( [ 'a', 'b' ] );
+		$this->assertSame( '<message key="key">' .
+			'<list listType="text">' .
+			'<text>a</text><text>b</text>' .
+			'</list></message>',
+			$mv->dump() );
+		$this->assertSame( $mv, $mv2 );
+	}
+}