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
Merge pull request #7273 from juho-jaakkola/load_translations_on_demand

fix(i18n): allow core to load translations for a specific language on-demand
fix(i18n): translate notification messages to the recipient's language