MediaWiki should support the language code "mul" to indicate multilingual content. This could be used as the content language on multilingual wikis, to avoid using a misleading "true" language code (as meta.wikimedia.org and commons.wikimedia.org currently do: they pretend to be in english, while they are not). This could also be used to indicate the language of truly multilingual pages, like item pages on wikidata.org.
The Language object for "mul" could be derived from the Language object for english - so it would inherit english messages, english date formatting, etc. I.e. sites that start to use "mul" as their content language instead of "en" would keep working exactly as before, except that they no longer lie about their content language.
Support for "mul" would also come in handy for language fallback mechanism: "mul" could act as the default fallback fro anything (instead of "en"). This is especially interesting for the fallback mechanism in wikibase, which (unlike mediawiki's i18n system) can not assume that english language messages exist.