You have a collection of ideas bundled together here. Separately they may be simple, but together they make for quite the extensive project!
(1) and (2) are definitely plugin territory.
(3) might also be plugin territory, but imo it would be beneficial to figure this one out for core, as I fully agree that it ought to be easier to discover and use our language switcher. @simon will soon be working on some localisation improvements for us. Maybe we can look into this then.
(4) Proper multi-lingual category names & descriptions ought to be in core at some point imo.
Translateable topic names on the other hand is very niche. This is plugin territory.