Mon (mnw) was unnecessarily added as a language for monolingual text (see T155420) and should be removed again. Mon is now available as an interface language and is therefore automatically usable in monolingual text statements.
I don't see any more where the codes match.
There is one that doesn't look right though: shy was added in T184783 for monolingual text and the request is clearly for Latin script, but https://translatewiki.net/wiki/Portal:Shy seems to have Arabic script as the default with shy-latn for Latin script. Neither code is in the language-data database. After shy was added for monolingual text, shy-latn was added to Names.php (T196473) because someone said on the first ticket that the code still didn't work for adding labels.