In SpecialNewLexeme, we need to make sure there is no confusion between the lemma language. In particular, the labels, names, and ids of the form fields need to be distinct and easily recognizable. I suggest to use "lexeme-language-item" and "lemma-language-code".
Note that the form field for the lemma language may no longer be needed once T154316: Special:NewLexeme should use derive lemma language from lexeme language. is implemented.