For monolingual text values, we currently support the language codes known to MediaWiki (416 language codes on 2015-01-26) plus 4 special ISO 639-2 codes (und, mis, mul, zxx).
As a user, I want to:
- Be able to look at no longer valid data (language code was in use previously but has been removed since)
- Neither see nor be able to use bad language codes while editing
- Be able to use all the language codes I need and be able to easily request missing language codes