diff --git a/.phpcs.xml b/.phpcs.xml
index 20e009955ee..30bb01c185e 100644
--- a/.phpcs.xml
+++ b/.phpcs.xml
@@ -180,15 +180,6 @@
 	<rule ref="MediaWiki.Usage.AssignmentInReturn.AssignmentInReturn">
 		<exclude-pattern>*/tests/phpunit/*</exclude-pattern>
 	</rule>
-	<rule ref="MediaWiki.Usage.ForbiddenFunctions.escapeshellarg">
-		<!--
-			Continue to allow existing violations, but enable the sniff to prevent
-			any new occurrences.
-		-->
-		<exclude-pattern>*/includes/libs/filebackend/FSFileBackend\.php</exclude-pattern>
-		<exclude-pattern>*/includes/shell/Command\.php</exclude-pattern>
-		<exclude-pattern>*/includes/shell/Shell\.php</exclude-pattern>
-	</rule>
 	<rule ref="MediaWiki.Usage.ForbiddenFunctions.popen">
 		<!--
 			Continue to allow existing violations, but enable the sniff to prevent
diff --git a/RELEASE-NOTES-1.38 b/RELEASE-NOTES-1.38
index 5250d7a6247..f09e9e72080 100644
--- a/RELEASE-NOTES-1.38
+++ b/RELEASE-NOTES-1.38
@@ -33,6 +33,11 @@ of automatic detection of possible phone numbers in a webpage in Safari on iOS.
 * …
 
 ==== Removed configuration ====
+* $wgShellLocale - This setting has been removed as it was a flawed solution
+  to the problem of locale dependence, MediaWiki will now always set a locale
+  of C.UTF-8 or C and works around the remaining problems of the C locale by
+  not using escapeshellarg. This follows the direction of PHP 8.0, which sets
+  a locale of C by default instead of respecting LC_CTYPE.
 * …
 
 === New user-facing features in 1.38 ===
diff --git a/includes/DefaultSettings.php b/includes/DefaultSettings.php
index d281a03191f..ebdc9a215cf 100644
--- a/includes/DefaultSettings.php
+++ b/includes/DefaultSettings.php
@@ -9535,42 +9535,6 @@ $wgShellCgroup = false;
  */
 $wgPhpCli = '/usr/bin/php';
 
-/**
- * Locale for LC_ALL, to provide a known environment for locale-sensitive operations
- *
- * For Unix-like operating systems, this should be set to C.UTF-8 or an
- * equivalent to provide the most consistent behavior for locale-sensitive
- * C library operations across different-language wikis. If that locale is not
- * available, use another locale that has a UTF-8 character set.
- *
- * This setting mainly affects the behavior of C library functions, including:
- *  - String collation (order when sorting using locale-sensitive comparison)
- *    - For example, whether "Å" and "A" are considered to be the same letter or
- *      different letters and if different whether it comes after "A" or after
- *      "Z", and whether sorting is case sensitive.
- *  - String character set (how characters beyond basic ASCII are represented)
- *    - We need this to be a UTF-8 character set to work around
- *      https://bugs.php.net/bug.php?id=45132
- *  - Language used for low-level error messages.
- *  - Formatting of date/time and numeric values (e.g. '.' versus ',' as the
- *    decimal separator)
- *
- * MediaWiki provides its own methods and classes to perform many
- * locale-sensitive operations, which are designed to be able to vary locale
- * based on wiki language or user preference:
- *  - MediaWiki's Collation class should generally be used instead of the C
- *    library collation functions when locale-sensitive sorting is needed.
- *  - MediaWiki's Message class should be used for localization of messages
- *    displayed to the user.
- *  - MediaWiki's Language class should be used for formatting numeric and
- *    date/time values.
- *
- * @note If multiple wikis are being served from the same process (e.g. the
- *  same fastCGI or Apache server), this setting must be the same on all those
- *  wikis.
- */
-$wgShellLocale = 'C.UTF-8';
-
 /**
  * Method to use to restrict shell commands
  *
diff --git a/includes/GlobalFunctions.php b/includes/GlobalFunctions.php
index 9d6d3e00929..35b09fe4a21 100644
--- a/includes/GlobalFunctions.php
+++ b/includes/GlobalFunctions.php
@@ -1870,11 +1870,12 @@ function wfStringToBool( $val ) {
 }
 
 /**
- * Version of escapeshellarg() that works better on Windows.
+ * Locale-independent version of escapeshellarg()
  *
  * Originally, this fixed the incorrect use of single quotes on Windows
  * (https://bugs.php.net/bug.php?id=26285) and the locale problems on Linux in
- * PHP 5.2.6+ (bug backported to earlier distro releases of PHP).
+ * PHP 5.2.6+ (https://bugs.php.net/bug.php?id=54391). The second bug is still
+ * open as of 2021.
  *
  * @param string|string[] ...$args strings to escape and glue together,
  *  or a single array of strings parameter
diff --git a/includes/Setup.php b/includes/Setup.php
index 85a99ff0975..89e95317294 100644
--- a/includes/Setup.php
+++ b/includes/Setup.php
@@ -169,10 +169,13 @@ ExtensionRegistry::getInstance()->loadFromQueue();
 // Don't let any other extensions load
 ExtensionRegistry::getInstance()->finish();
 
-// Set the configured locale on all requests for consistency
-// This must be after LocalSettings.php (and is informed by the installer).
-putenv( "LC_ALL=$wgShellLocale" );
-setlocale( LC_ALL, $wgShellLocale );
+// Set an appropriate locale (T291234)
+// setlocale() will return the locale name actually set.
+// The putenv() is meant to propagate the choice of locale to shell commands
+// so that they will interpret UTF-8 correctly. If you have a problem with a
+// shell command and need to send a special locale, you can override the locale
+// with Command::environment().
+putenv( "LC_ALL=" . setlocale( LC_ALL, 'C.UTF-8', 'C' ) );
 
 /**
  * Expand dynamic defaults and shortcuts
diff --git a/includes/installer/Installer.php b/includes/installer/Installer.php
index 31cab5c8c83..0e46afd7e88 100644
--- a/includes/installer/Installer.php
+++ b/includes/installer/Installer.php
@@ -28,7 +28,6 @@ use MediaWiki\HookContainer\HookContainer;
 use MediaWiki\HookContainer\StaticHookRegistry;
 use MediaWiki\Interwiki\NullInterwikiLookup;
 use MediaWiki\MediaWikiServices;
-use MediaWiki\Shell\Shell;
 
 /**
  * The Installer helps admins create or upgrade their wiki.
@@ -144,7 +143,6 @@ abstract class Installer {
 		'envCheckGit',
 		'envCheckServer',
 		'envCheckPath',
-		'envCheckShellLocale',
 		'envCheckUploadsDirectory',
 		'envCheckLibicu',
 		'envCheckSuhosinMaxValueLength',
@@ -191,7 +189,6 @@ abstract class Installer {
 		'wgMetaNamespace',
 		'wgDeletedDirectory',
 		'wgEnableUploads',
-		'wgShellLocale',
 		'wgSecretKey',
 		'wgUseInstantCommons',
 		'wgUpgradeKey',
@@ -1011,87 +1008,6 @@ abstract class Installer {
 		return true;
 	}
 
-	/**
-	 * Environment check for preferred locale in shell
-	 * @return bool
-	 */
-	protected function envCheckShellLocale() {
-		$os = php_uname( 's' );
-		$supported = [ 'Linux', 'SunOS', 'HP-UX', 'Darwin' ]; # Tested these
-
-		if ( !in_array( $os, $supported ) ) {
-			return true;
-		}
-
-		if ( Shell::isDisabled() ) {
-			return true;
-		}
-
-		# Get a list of available locales.
-		$result = Shell::command( '/usr/bin/locale', '-a' )->execute();
-
-		if ( $result->getExitCode() != 0 ) {
-			return true;
-		}
-
-		$lines = $result->getStdout();
-		$lines = array_map( 'trim', explode( "\n", $lines ) );
-		$candidatesByLocale = [];
-		$candidatesByLang = [];
-		foreach ( $lines as $line ) {
-			if ( $line === '' ) {
-				continue;
-			}
-
-			if ( !preg_match( '/^([a-zA-Z]+)(_[a-zA-Z]+|)\.(utf8|UTF-8)(@[a-zA-Z_]*|)$/i', $line, $m ) ) {
-				continue;
-			}
-
-			list( , $lang, , , ) = $m;
-
-			$candidatesByLocale[$m[0]] = $m;
-			$candidatesByLang[$lang][] = $m;
-		}
-
-		# Try the current value of LANG.
-		if ( isset( $candidatesByLocale[getenv( 'LANG' )] ) ) {
-			$this->setVar( 'wgShellLocale', getenv( 'LANG' ) );
-
-			return true;
-		}
-
-		# Try the most common ones.
-		$commonLocales = [ 'C.UTF-8', 'en_US.UTF-8', 'en_US.utf8', 'de_DE.UTF-8', 'de_DE.utf8' ];
-		foreach ( $commonLocales as $commonLocale ) {
-			if ( isset( $candidatesByLocale[$commonLocale] ) ) {
-				$this->setVar( 'wgShellLocale', $commonLocale );
-
-				return true;
-			}
-		}
-
-		# Is there an available locale in the Wiki's language?
-		$wikiLang = $this->getVar( 'wgLanguageCode' );
-
-		if ( isset( $candidatesByLang[$wikiLang] ) ) {
-			$m = reset( $candidatesByLang[$wikiLang] );
-			$this->setVar( 'wgShellLocale', $m[0] );
-
-			return true;
-		}
-
-		# Are there any at all?
-		if ( count( $candidatesByLocale ) ) {
-			$m = reset( $candidatesByLocale );
-			$this->setVar( 'wgShellLocale', $m[0] );
-
-			return true;
-		}
-
-		# Give up.
-		return true;
-	}
-
 	/**
 	 * Environment check for the permissions of the uploads directory
 	 * @return bool
diff --git a/includes/installer/LocalSettingsGenerator.php b/includes/installer/LocalSettingsGenerator.php
index 6f4c9fef700..656ade6166b 100644
--- a/includes/installer/LocalSettingsGenerator.php
+++ b/includes/installer/LocalSettingsGenerator.php
@@ -56,7 +56,7 @@ class LocalSettingsGenerator {
 		$confItems = array_merge(
 			[
 				'wgServer', 'wgScriptPath',
-				'wgPasswordSender', 'wgImageMagickConvertCommand', 'wgShellLocale',
+				'wgPasswordSender', 'wgImageMagickConvertCommand',
 				'wgLanguageCode', 'wgLocaltimezone', 'wgEnableEmail', 'wgEnableUserEmail',
 				'wgDiff3', 'wgEnotifUserTalk', 'wgEnotifWatchlist', 'wgEmailAuthentication',
 				'wgDBtype', 'wgSecretKey', 'wgRightsUrl', 'wgSitename', 'wgRightsIcon',
@@ -242,13 +242,6 @@ class LocalSettingsGenerator {
 			$magic = '';
 		}
 
-		if ( !$this->values['wgShellLocale'] ) {
-			$this->values['wgShellLocale'] = 'C.UTF-8';
-			$locale = '#';
-		} else {
-			$locale = '';
-		}
-
 		$metaNamespace = '';
 		if ( $this->values['wgMetaNamespace'] !== $this->values['wgSitename'] ) {
 			$metaNamespace = "\$wgMetaNamespace = \"{$this->values['wgMetaNamespace']}\";\n";
@@ -392,14 +385,6 @@ ${serverSetting}
 # with MediaWiki developers to help guide future development efforts.
 \$wgPingback = {$this->values['wgPingback']};
 
-## If you use ImageMagick (or any other shell command) on a
-## Linux server, this will need to be set to the name of an
-## available UTF-8 locale. This should ideally be set to an English
-## language locale so that the behaviour of C library functions will
-## be consistent with typical installations. Use \$wgLanguageCode to
-## localise the wiki.
-{$locale}\$wgShellLocale = \"{$this->values['wgShellLocale']}\";
-
 # Site language code, should be one of the list in ./languages/data/Names.php
 \$wgLanguageCode = \"{$this->values['wgLanguageCode']}\";
 
diff --git a/includes/libs/ParamValidator/TypeDef/FloatDef.php b/includes/libs/ParamValidator/TypeDef/FloatDef.php
index 53f1c0efa71..c90cb5ca78e 100644
--- a/includes/libs/ParamValidator/TypeDef/FloatDef.php
+++ b/includes/libs/ParamValidator/TypeDef/FloatDef.php
@@ -44,32 +44,10 @@ class FloatDef extends NumericDef {
 		return $this->checkRange( $ret, $name, $value, $settings, $options );
 	}
 
-	/**
-	 * Attempt to fix locale weirdness
-	 *
-	 * We don't have any usable number formatting function that's not locale-aware,
-	 * and `setlocale()` isn't safe in multithreaded environments. Sigh.
-	 *
-	 * @param string $value Value to fix
-	 * @return string
-	 */
-	private function fixLocaleWeirdness( $value ) {
-		$localeData = localeconv();
-		if ( $localeData['decimal_point'] !== '.' ) {
-			$value = strtr( $value, [
-				$localeData['decimal_point'] => '.',
-				// PHP's number formatting currently uses only the first byte from 'decimal_point'.
-				// See upstream bug https://bugs.php.net/bug.php?id=78113
-				$localeData['decimal_point'][0] => '.',
-			] );
-		}
-		return $value;
-	}
-
 	public function stringifyValue( $name, $value, array $settings, array $options ) {
 		// Ensure sufficient precision for round-tripping. PHP_FLOAT_DIG was added in PHP 7.2.
 		$digits = defined( 'PHP_FLOAT_DIG' ) ? PHP_FLOAT_DIG : 15;
-		return $this->fixLocaleWeirdness( sprintf( "%.{$digits}g", $value ) );
+		return sprintf( "%.{$digits}g", $value );
 	}
 
 	public function getHelpInfo( $name, array $settings, array $options ) {
diff --git a/includes/libs/filebackend/FSFileBackend.php b/includes/libs/filebackend/FSFileBackend.php
index 7b9676c5e79..c3a807093d7 100644
--- a/includes/libs/filebackend/FSFileBackend.php
+++ b/includes/libs/filebackend/FSFileBackend.php
@@ -41,6 +41,7 @@
  * @ingroup FileBackend
  */
 
+use Shellbox\Shellbox;
 use Wikimedia\AtEase\AtEase;
 use Wikimedia\Timestamp\ConvertibleTimestamp;
 
@@ -822,9 +823,9 @@ class FSFileBackend extends FileBackendStore {
 		// inode are unaffected since it writes to a new inode, and (c) new threads reading
 		// the file will either totally see the old version or totally see the new version
 		$fsStagePath = $this->makeStagingPath( $fsDstPath );
-		$encSrc = escapeshellarg( $this->cleanPathSlashes( $fsSrcPath ) );
-		$encStage = escapeshellarg( $this->cleanPathSlashes( $fsStagePath ) );
-		$encDst = escapeshellarg( $this->cleanPathSlashes( $fsDstPath ) );
+		$encSrc = Shellbox::escape( $this->cleanPathSlashes( $fsSrcPath ) );
+		$encStage = Shellbox::escape( $this->cleanPathSlashes( $fsStagePath ) );
+		$encDst = Shellbox::escape( $this->cleanPathSlashes( $fsDstPath ) );
 		if ( $this->os === 'Windows' ) {
 			// https://docs.microsoft.com/en-us/windows-server/administration/windows-commands/copy
 			// https://docs.microsoft.com/en-us/windows-server/administration/windows-commands/move
@@ -854,8 +855,8 @@ class FSFileBackend extends FileBackendStore {
 	private function makeMoveCommand( $fsSrcPath, $fsDstPath, $ignoreMissing = false ) {
 		// https://manpages.debian.org/buster/coreutils/mv.1.en.html
 		// https://docs.microsoft.com/en-us/windows-server/administration/windows-commands/move
-		$encSrc = escapeshellarg( $this->cleanPathSlashes( $fsSrcPath ) );
-		$encDst = escapeshellarg( $this->cleanPathSlashes( $fsDstPath ) );
+		$encSrc = Shellbox::escape( $this->cleanPathSlashes( $fsSrcPath ) );
+		$encDst = Shellbox::escape( $this->cleanPathSlashes( $fsDstPath ) );
 		if ( $this->os === 'Windows' ) {
 			$writeCmd = "MOVE /Y $encSrc $encDst 2>&1";
 			$cmd = $ignoreMissing ? "IF EXIST $encSrc $writeCmd" : $writeCmd;
@@ -875,7 +876,7 @@ class FSFileBackend extends FileBackendStore {
 	private function makeUnlinkCommand( $fsPath, $ignoreMissing = false ) {
 		// https://manpages.debian.org/buster/coreutils/rm.1.en.html
 		// https://docs.microsoft.com/en-us/windows-server/administration/windows-commands/del
-		$encSrc = escapeshellarg( $this->cleanPathSlashes( $fsPath ) );
+		$encSrc = Shellbox::escape( $this->cleanPathSlashes( $fsPath ) );
 		if ( $this->os === 'Windows' ) {
 			$writeCmd = "DEL /Q $encSrc 2>&1";
 			$cmd = $ignoreMissing ? "IF EXIST $encSrc $writeCmd" : $writeCmd;
diff --git a/includes/parser/Parser.php b/includes/parser/Parser.php
index df3a0f9093c..b35c3c765be 100644
--- a/includes/parser/Parser.php
+++ b/includes/parser/Parser.php
@@ -4338,7 +4338,6 @@ class Parser {
 			}
 
 			# HTML IDs must be case-insensitively unique for IE compatibility (T12721).
-			# @todo FIXME: We may be changing them depending on the current locale.
 			$arrayKey = strtolower( $safeHeadline );
 			if ( $fallbackHeadline === false ) {
 				$fallbackArrayKey = false;
diff --git a/includes/shell/Shell.php b/includes/shell/Shell.php
index 51ce2398d73..9253649f876 100644
--- a/includes/shell/Shell.php
+++ b/includes/shell/Shell.php
@@ -151,11 +151,12 @@ class Shell {
 	}
 
 	/**
-	 * Version of escapeshellarg() that works better on Windows.
+	 * Locale-independent version of escapeshellarg()
 	 *
 	 * Originally, this fixed the incorrect use of single quotes on Windows
 	 * (https://bugs.php.net/bug.php?id=26285) and the locale problems on Linux in
-	 * PHP 5.2.6+ (bug backported to earlier distro releases of PHP).
+	 * PHP 5.2.6+ (https://bugs.php.net/bug.php?id=54391). The second bug is still
+	 * open as of 2021.
 	 *
 	 * @param string|string[] ...$args strings to escape and glue together, or a single
 	 *     array of strings parameter. Null values are ignored.
diff --git a/tests/phpunit/unit/includes/libs/ParamValidator/TypeDef/FloatDefTest.php b/tests/phpunit/unit/includes/libs/ParamValidator/TypeDef/FloatDefTest.php
index c80db45ce65..63163e93a27 100644
--- a/tests/phpunit/unit/includes/libs/ParamValidator/TypeDef/FloatDefTest.php
+++ b/tests/phpunit/unit/includes/libs/ParamValidator/TypeDef/FloatDefTest.php
@@ -154,53 +154,6 @@ class FloatDefTest extends TypeDefTestCase {
 		];
 	}
 
-	/** @dataProvider provideLocales */
-	public function testStringifyValue_localeWeirdness( $locale ) {
-		static $cats = [ LC_ALL, LC_MONETARY, LC_NUMERIC ];
-
-		$curLocales = [];
-		foreach ( $cats as $c ) {
-			$curLocales[$c] = setlocale( $c, '0' );
-			if ( $curLocales[$c] === false ) {
-				$this->markTestSkipped( 'Locale support is unavailable' );
-			}
-		}
-		try {
-			foreach ( $cats as $c ) {
-				if ( setlocale( $c, $locale ) === false ) {
-					$this->markTestSkipped( "Locale \"$locale\" is unavailable" );
-				}
-			}
-
-			$typeDef = $this->getInstance( new SimpleCallbacks( [] ), [] );
-			$this->assertSame( '123456.789', $typeDef->stringifyValue( 'test', 123456.789, [], [] ) );
-			$this->assertSame( '-123456.789', $typeDef->stringifyValue( 'test', -123456.789, [], [] ) );
-			$this->assertSame( '1.0e+20', $typeDef->stringifyValue( 'test', 1e20, [], [] ) );
-			$this->assertSame( '1.0e-20', $typeDef->stringifyValue( 'test', 1e-20, [], [] ) );
-		} finally {
-			foreach ( $curLocales as $c => $v ) {
-				setlocale( $c, $v );
-			}
-		}
-	}
-
-	public function provideLocales() {
-		return [
-			// May as well test these.
-			[ 'C' ],
-			[ 'C.UTF-8' ],
-
-			// Some hopefullt-common locales with decimal_point = ',' and thousands_sep = '.'
-			[ 'de_DE' ],
-			[ 'de_DE.utf8' ],
-			[ 'es_ES' ],
-			[ 'es_ES.utf8' ],
-
-			// This one, on my system at least, has decimal_point as U+066B.
-			[ 'ps_AF' ],
-		];
-	}
-
 	public function provideGetInfo() {
 		return [
 			'Basic test' => [