NewLexeme: allow lexeme creation with 'mis' language
Allow editors to create lexemes (through Special:NewLexeme) with a
lemma bearing the special 'mis' language code - as already possible when
editing lexemes on the lexeme detail page.
Removes the infering of lemma language default from the form (user)
language (only partially working, not documented).
Bug: T194771
Change-Id: Ia1d2d0e78cbd078972bad84fb2c166b3e18b11b5