mw.Platform: Use 'mediawiki.language' for language fallback chain

Bonus:
* Remove doubled dependency on mediawiki.Uri
* Add missing dependency on mediawiki.language

Change-Id: Ide716aad7b9f08ae9a24f99812f07273d89da33a
Merge "mw.Platform: Use 'mediawiki.language' for language fallback chain"