Could be a use case for Lua, but not as a parser function. Wikitext is not PHP.

Of course, actually implementing this is rather easy... Something like:

/**

• Reports whether language code is valid.
• @param $parser Parser
• @param $inStr string
• @return bool

	 */

public static function runValidLanguagecode ( $parser, $inStr = '' ) {

		wfProfileIn( __METHOD__ );

		$inStr = self::killMarkers( $parser, (string)$inStr );
		$valid = Language::isValidCode( $inStr );

		wfProfileOut( __METHOD__ );
		return $valid;

}

It could go in string functions and it could also go into core... That's the hard part

(In reply to comment #2)

Of course, actually implementing this is rather easy...

I'm not sure it was, but it should be now that bug 41477 was fixed.

(In reply to Siebrand Mazeland from comment #1)

Could be a use case for Lua, but not as a parser function. Wikitext is not
PHP.

Indeed. In fact, Lua already has mw.language.isValidCode (and a few variants) designed to do just this, so I see no reason we should add a parser function to do it.

(In reply to Jackmcbarn from comment #4)

I see no reason we should add a
parser function to do it.

Well, the reason is explained in comment 0. :-) However the use case was covered in Translate with bug 68385 now and templates based on this implementation would be hacky anyway, so thanks for closing this.