It boils down to which you prefer. Using the same instance, you can
- Create individual categories that are language-specific as you’ve described, or
- Use the
set locale from accept language header
site setting to have to your users register on the site with the locale their browser is set to.
I see you already asked a question over at How to structure a multilingual community - #34 by Yves_Lauzin, so no need to mention that post again.