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* Be able to look at no longer valid data
* 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