chore(tests): throw ElggUpgrade exceptions untranslated to get rid of library dependencies

ElggUpgrade was translating its exception messages which required
ElggUpgradeTest to include the languages library. Use of the libraries
is prone to cause dependency chains, so it's better not to translate
the exceptions at all for now.

Fixes #7276
Merge pull request #7277 from juho-jaakkola/dont_translate_upgrade_exceptions

chore(tests): throw ElggUpgrade exceptions untranslated to get rid of library dependencies
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