If I go to user settings > interface on meta.discourse.org, I get a language selector. I don’t see that here. Does it need to be enabled somehow?
Once that’s fixed, there should be some sort of autodetection where users with a non-english browser language get a banner or similar notice about changing the language. Not sure if Discourse supports that out of the box, but IMO it’s part of the bare minimum feature set for any multilingual website, and worth paying for if necessary.