chore(i18n): Refactor i18n code into OO service

This is based on and replaces @cash's original work in https://github.com/Elgg/Elgg/pull/5663

TODO:
 - [x] Requires #7132
 - [ ] Add tests
   - [ ] `Elgg\I18n\Loader`
   - [ ] `elgg_echo`
   - [ ] `add_translation`
   - [ ] `get_language`