In WikibaseContentLanguages.php there is a list of language codes which shouldn't be allowed for monolingual text. "als" should be added to this list.

MediaWiki uses "als" to mean Alemannic, which can already be entered using the standard code "gsw".

As a standard code, "als" means Tosk Albanian. It is occasionally used for that in Wikidata (e.g. https://www.wikidata.org/w/index.php?title=Q180937&oldid=1910632541) but I don't think that's a good idea because we have no way of knowing whether someone is using it to mean Tosk Albanian or to mean Alemannic. Such statements can be entered using "sq" (Albanian), and if we want a specific option for Tosk Albanian, adding something like "als-latn" would probably be a better option.