Yes. For the tag-panel fields I'd check the default ones (currently ALBUMARTIST, COMPOSER, and DISCNUMBER) and use the translated names for them on language change. Same applies to the columns of the file view.
I'm not sure, if I fully understand what you mean by "special name". I'd look at the internal representation which is the field name that is used internally (same as, e.g., in format strings) and use this one as a means for detecting "original" fields. Then compare the language-specific name of this default field (e.g., Kompozitorius for composer in Lithuanian) and only then auto-translate to the target language.
No. If you add a "real" user-defined entry to the Tag Panel, e.g., "Sortierung Interpret" for ARTISTSORT, Mp3tag would leave this untouched.
— Florian