It points to https://www.wikidata.org/wiki/Special:MyLanguage/Help:Monolingual_text_languages but that page said nothing about lemma language code.
- go to https://www.wikidata.org/wiki/Special:NewLexeme
- enter non language to lexeme language
- see the (help) link that appears in the 'Spelling variant of the Lemma' field
To do
- provide correct link