Changeable UI language in SA demo

Re-initialize the platform when the language is changed.

Bonus:
* Override OO.ui.getUserLanguages like we do in MW.
* Cache user languages on initialize and return in getUserLanguages
  instead of attempting to duplicate $.i18n's logic.
* Hackiness to support qqx mode.

Future:
* Provide a clean API for overriding the UI language.

Change-Id: Ie83122e8abc0cb129e7d02b9cc46332503746982
Merge "Changeable UI language in SA demo"