Currently MediaWiki contains a mapping of deprecated language codes:

	private static $deprecatedLanguageCodeMapping = [
		// 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', // T25215
		'bat-smg' => 'sgs', // T27522
		'be-x-old' => 'be-tarask', // T11823
		'fiu-vro' => 'vro', // T31186
		'roa-rup' => 'rup', // T17988
		'zh-classical' => 'lzh', // T30443
		'zh-min-nan' => 'nan', // T30442
		'zh-yue' => 'yue', // T30441
	];

The language code als is only WMF specific because the Alemannic Wikipedia was created at https://als.wikipedia.org/. The Alemannic Wikipedia uses as language code gsw with the language names Alemannic, Alsatian, Swiss German (https://iso639-3.sil.org/code/gsw). als is the language code for Tosk Albanian (https://iso639-3.sil.org/code/als).

The current mapping in MediaWiki blocks that als can used as Tosk Albanian in a MediaWiki project not related to the WMF projects. The mapping is only specific to WMF projects and therefor the mapping from als to gsw should moved out of MediaWiki into a WMF configuration for the WMF projects.