We allow editors to use mis as a language code for entering languages we don't have in our list. However we only allow this on the Lexeme page, not when creating a new Lexeme via Special:NewLexeme. This should be possible there too.
GIVEN I am on Special:NewLexeme page AND I pick a language item without the language code property set WHEN I open the lemma language dropdown THEN There is a "Other (mis)" option in the list
Note: this does not include extended language codes such as mis-x-Q321