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 1: Don't document the parameter in 1.9 (note why in comments, not in phpdoc), submit a simultaneous PR on 1.x to document the param. I think this is a decent solution given this function is rarely used in plugins, but I'm not sure it obeys the letter of SemVer.

Idea 2: Pass the state in via a global $_ELGG_REGISTER_TRANSLATIONS_LANG_OVERRIDE. Submit a 1.x PR replacing this global with the added parameter.