This is one thing that caught my attention when setting up two locales in esoTalk: that channel names and group names could only ever be in one language. (The forum name didn't make the list, since I replaced it with an image.)
We've got translation of the default user group names covered in Flarum, but localization of admin-added group names would be something to think about. Same goes for tags, of course. The big question is how to make room for those translations in the Edit Group/Tags extension admin UI and the database? And what happens to the localized group or tag names when the language pack is disabled/removed? If you can think of a good way to deal with those issues, then it would be a neat feature to have.
That's assuming that we're just talking about localizing how the group/tag name is displayed in the UI, and the slug remains the same. Another possibility would be to give flare a truly multilingual framework, so that each group or tag name gets associated with a locale, so that the user's choice of locale can determine what content is available. In that case we'd need to add the locale to the URL ... but it wouldn't be necessary to associate multiple tag names with a single slug, so a bit easier in that sense. We'd still have to do something about group names, though.
(WoltLab's Burning Board is probably the best example of this sort of thing in a forum script. But I'm not sure how much of a demand there is for that sort of forum.)
One concern I would have about translating tag names is that it could make it difficult to settle on a lingua franca for a given tag. Users will see the tag name in their chosen language and assume that they're welcome to post in that language. That's fine for sites that don't mind posts in many languages, of course; but we'd probably want to avoid doing that in this forum. Of course, the software won't actually force anyone to localize tag names, so I suppose they'll just use the default name (English, in our case) if the localization fields are left blank...
Which brings us to the Easy Way Out™. How often will forum admins really want to localize tag names? Is it more likely that they'll want to provide separate spaces for each language? If the latter is overwhelmingly more likely, then we've already got it covered: a single forum can have a few tags in English and a few in French (or whatever) which will give users an idea of what language they should be posting in each tag.
... But even there, it would still be a good idea to make admin-created group names localizable.
As for the forum title, it might be nice if it could be displayed in more than one langue in the UI. But I think we'd want to use the default language only in the site metadata, for SEO reasons.