The combobox selector shown when selecting a language for a monolingual text value does not list special languages like "mis" as supported. Entering the code blindly works, however. The selector should at least show the language codes, and if ideally also a name for these special languages (doesn't CLDR have names for these?).
Special language codes, according to https://www.wikidata.org/wiki/Help:Monolingual_text_languages:
- und: For content whose language is not yet determined (undetermined)
- mis: For content whose language is known, but has no language code (uncoded languages); we also use it for content whose language has a language code, but is not yet available on Wikidata.org.
- mul: For content in multiple languages (multiple languages)
- zxx: For content that is not linguistic (no linguistic content, not applicable)