Update European Portuguese (pt_pt) translation to 100% completion.

Sweeping revamp of existing translation, not only bringing it to completion, but also improving previous work, while updating many of the existing strings to fix any shortcomings and downright incorrect translations.

This commit represents the collective work of all participants, and was done in large part thanks to the persistence of @AnaCG https://gitlab.com/AnaCG, whose tireless efforts single-handedly completed the bulk of the work, with some minor contributions from myself.