If you go to a page and specify an non-existing content language https://test.wikidata.org/w/index.php?title=Q12345&uselang=xyzzy, the interface will offer you to enter the label in the language. This will fail with "Unrecognized value for parameter 'language': xyzzy" error, but I believe the interface should not even offer the possibility to enter the label, perhaps displaying an error right away.
See Also:
T41623: Invalid language codes via uselang are used for lang HTML tag et al.
T44396: duplicate/invalid language codes
T51024: [Task] Removal of de-formal from allowed language for labels
T38430: Specify language fallback
T66649: 'Add link' function in Wikipedia creates items with wrong language code