diff --git a/autoload.php b/autoload.php
index f96d8986630..0f793238261 100644
--- a/autoload.php
+++ b/autoload.php
@@ -682,6 +682,7 @@ $wgAutoloadLocalClasses = [
 	'LanguageBe_tarask' => __DIR__ . '/languages/classes/LanguageBe_tarask.php',
 	'LanguageBg' => __DIR__ . '/languages/classes/LanguageBg.php',
 	'LanguageBs' => __DIR__ . '/languages/classes/LanguageBs.php',
+	'LanguageCode' => __DIR__ . '/languages/LanguageCode.php',
 	'LanguageConverter' => __DIR__ . '/languages/LanguageConverter.php',
 	'LanguageCu' => __DIR__ . '/languages/classes/LanguageCu.php',
 	'LanguageDsb' => __DIR__ . '/languages/classes/LanguageDsb.php',
diff --git a/includes/DefaultSettings.php b/includes/DefaultSettings.php
index a3d68c6bfea..add4bfa12d5 100644
--- a/includes/DefaultSettings.php
+++ b/includes/DefaultSettings.php
@@ -2825,8 +2825,9 @@ $wgUsePrivateIPs = false;
  * MediaWiki out of the box. Not all languages listed there have translations,
  * see languages/messages/ for the list of languages with some localisation.
  *
- * Warning: Don't use language codes listed in $wgDummyLanguageCodes like "no"
- * for Norwegian (use "nb" instead), or things will break unexpectedly.
+ * Warning: Don't use any of MediaWiki's deprecated language codes listed in
+ * LanguageCode::getDeprecatedCodeMapping or $wgDummyLanguageCodes, like "no"
+ * for Norwegian (use "nb" instead). If you do, things will break unexpectedly.
  *
  * This defines the default interface language for all users, but users can
  * change it in their preferences.
@@ -2885,25 +2886,22 @@ $wgExtraInterlanguageLinkPrefixes = [];
 $wgExtraLanguageNames = [];
 
 /**
- * List of language codes that don't correspond to an actual language.
- * These codes are mostly left-offs from renames, or other legacy things.
- * This array makes them not appear as a selectable language on the installer,
- * and excludes them when running the transstat.php script.
+ * List of mappings from one language code to another.
+ * This array makes the codes not appear as a selectable language on the
+ * installer, and excludes them when running the transstat.php script.
+ *
+ * In Setup.php, the variable $wgDummyLanguageCodes is created by combining
+ * these codes with a list of "deprecated" codes, which are mostly leftovers
+ * from renames or other legacy things, and the internal codes 'qqq' and 'qqx'.
+ * If a mapping in $wgExtraLanguageCodes collide with a built-in mapping, the
+ * value in $wgExtraLanguageCodes will be used.
+ *
+ * @since 1.29
  */
-$wgDummyLanguageCodes = [
-	'als' => 'gsw',
-	'bat-smg' => 'sgs',
-	'be-x-old' => 'be-tarask',
-	'bh' => 'bho',
-	'fiu-vro' => 'vro',
-	'no' => 'nb',
-	'qqq' => 'qqq', # Used for message documentation.
-	'qqx' => 'qqx', # Used for viewing message keys.
-	'roa-rup' => 'rup',
-	'simple' => 'en',
-	'zh-classical' => 'lzh',
-	'zh-min-nan' => 'nan',
-	'zh-yue' => 'yue',
+$wgExtraLanguageCodes = [
+	'bh' => 'bho', // Bihari language family
+	'no' => 'nb', // Norwegian language family
+	'simple' => 'en', // Simple English
 ];
 
 /**
diff --git a/includes/Setup.php b/includes/Setup.php
index b61de733769..e686cd81d6e 100644
--- a/includes/Setup.php
+++ b/includes/Setup.php
@@ -403,6 +403,14 @@ if ( is_array( $wgExtraNamespaces ) ) {
 	$wgCanonicalNamespaceNames = $wgCanonicalNamespaceNames + $wgExtraNamespaces;
 }
 
+// Merge in the legacy language codes, unless overridden in the config
+if ( !isset( $wgDummyLanguageCodes ) ) {
+	$wgDummyLanguageCodes = [
+		'qqq' => 'qqq', // Used for message documentation
+		'qqx' => 'qqx', // Used for viewing message keys
+	] + $wgExtraLanguageCodes + LanguageCode::getDeprecatedCodeMapping();
+}
+
 // These are now the same, always
 // To determine the user language, use $wgLang->getCode()
 $wgContLanguageCode = $wgLanguageCode;
diff --git a/languages/LanguageCode.php b/languages/LanguageCode.php
new file mode 100644
index 00000000000..7c9da77ba61
--- /dev/null
+++ b/languages/LanguageCode.php
@@ -0,0 +1,54 @@
+<?php
+/**
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ * http://www.gnu.org/copyleft/gpl.html
+ *
+ * @file
+ * @ingroup Language
+ */
+
+/**
+ * Methods for dealing with language codes.
+ * @todo Move some of the code-related static methods out of Language into this class
+ *
+ * @since 1.29
+ * @ingroup Language
+ */
+class LanguageCode {
+	/**
+	 * Returns a mapping of deprecated language codes that were used in previous
+	 * versions of MediaWiki to up-to-date, current language codes.
+	 *
+	 * This array is merged into $wgDummyLanguageCodes in Setup.php, along with
+	 * the fake language codes 'qqq' and 'qqx', which are used internally by
+	 * MediaWiki's localisation system.
+	 *
+	 * @return string[]
+	 */
+	public static function getDeprecatedCodeMapping() {
+		return [
+			// Note that als is actually a valid ISO 639 code (Tosk Albanian), but it
+			// was previously used in MediaWiki for Alsatian, which comes under gsw
+			'als' => 'gsw',
+			'bat-smg' => 'sgs',
+			'be-x-old' => 'be-tarask',
+			'fiu-vro' => 'vro',
+			'roa-rup' => 'rup',
+			'zh-classical' => 'lzh',
+			'zh-min-nan' => 'nan',
+			'zh-yue' => 'yue',
+		];
+	}
+}
diff --git a/tests/phpunit/languages/LanguageCodeTest.php b/tests/phpunit/languages/LanguageCodeTest.php
new file mode 100644
index 00000000000..b33360cfe9f
--- /dev/null
+++ b/tests/phpunit/languages/LanguageCodeTest.php
@@ -0,0 +1,40 @@
+<?php
+
+/**
+ * @covers LanguageCode
+ *
+ * @group Language
+ *
+ * @license GPL-2.0+
+ * @author Thiemo Mättig
+ */
+class LanguageCodeTest extends PHPUnit_Framework_TestCase {
+
+	public function testConstructor() {
+		$instance = new LanguageCode();
+
+		$this->assertInstanceOf( LanguageCode::class, $instance );
+	}
+
+	public function testGetDeprecatedCodeMapping() {
+		$map = LanguageCode::getDeprecatedCodeMapping();
+
+		$this->assertInternalType( 'array', $map );
+		$this->assertContainsOnly( 'string', array_keys( $map ) );
+		$this->assertArrayNotHasKey( '', $map );
+		$this->assertContainsOnly( 'string', $map );
+		$this->assertNotContains( '', $map );
+
+		// Codes special to MediaWiki should never appear in a map of "deprecated" codes
+		$this->assertArrayNotHasKey( 'qqq', $map, 'documentation' );
+		$this->assertNotContains( 'qqq', $map, 'documentation' );
+		$this->assertArrayNotHasKey( 'qqx', $map, 'debug code' );
+		$this->assertNotContains( 'qqx', $map, 'debug code' );
+
+		// Valid language codes that are currently not "deprecated"
+		$this->assertArrayNotHasKey( 'bh', $map, 'family of Bihari languages' );
+		$this->assertArrayNotHasKey( 'no', $map, 'family of Norwegian languages' );
+		$this->assertArrayNotHasKey( 'simple', $map );
+	}
+
+}