Fix missing locale entries in locale/analyse

By default, fork's FrontendLanguage class merges the current language with English, to have a fallback. This behaviour doesn't allow you to have:

1. A correct listing of missing strings for languages other than English in backend/{lang}/locale/analyse
2. 'Missing' tags e.g.: {$lblChooseDestination} in the frontend, when in debug mode.

Those are really handy features for debugging.

This commit fixes that issue.