Followup for r81340:

Followup for r81340:
* Allow any language code which consists entirely of valid title characters, and does not contain any path-traversal characters, to be customised via the uselang parameter. Language::isValidCode() represents this concept.
* Add some shortcuts preventing Language and LocalisationCache from looking for localisation files for a language code which does not follow the usual form of language codes in MediaWiki, i.e. /[a-z-]*/. This concept is represented by Language::isValidBuiltInCode().
* Do not allow colon characters in file names, per Platonides' suggestion on CR.