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 
I assume this was copied from somewhere where this exception would be handled in a sane way. I think here it would likely dump the exception page, likely during an action. Should we catch this and do something else?