plug-ins: add and correct gettext macros in Python

This takes care of the lowest-hanging fruit of translatability issues in #9794 (closed). The other (bigger) parts need more thought.

Added translation macros, and replaced two N_ with _ so the translations actually are shown.

Off_set was interesting since it has mnemonics even though it's an unselectable label. This looks like an error to me, so removed the mnemonics. Or does it have some function I haven't figured out?