fix(i18n): allow core to load translations for a specific language on-demand

By default Elgg is loading only English and the language of the logged in user.
These two are not enough e.g. when we want to send a notification and the
recipient is using a third language. This fix makes it possible to load translations
for a specific language when necessary.

Refs #3151 
Idea 3. Just make a private duplicate of register_translations for 1.9 and replace it with the param in 1.x. I think the temporary duplication is the weakest sin.