There is a feature request for #1 here: Feature request: make it possible to translate custom sidebar links and sections. So I don’t think it’s currently possible. You can vote on it to show that it matters to you.
That said, I know custom badges can be translated via the rails console, but I haven’t tried if that also works for the sidebar. In case you want to try it, it was explained here: How can badges and groups be localized (multilingual)? - #8 by nat
Theme components support localization. There is a short guide about it here:
I usually prefer to look at examples. There are lots of theme components in Customization > Theme component. I think Topic Status Filter is a quite simple one. Custom Top Navigation Links might be a helpful example too. It’s closer to what you want to do, but it doesn’t support localizable links, because combining that with them being configurable is difficult.
I don’t know if something similar is possible directly in the <head> section.