The goal of having a discourse instance is to have a better tool to discuss inside the community than the mailing lists. But we got a barrier for completely replace the mailing list, this barrier is the language. We have mailing list for each languages. This discourse instance is English only for now. I think it will be good to allow different language on the instance but instead of creating barriers like the mailing list does (there are not inter-communication between languages mailing lists), we should try to encourage multi-lingual conversation because we are a community and we share the same issues and goals.
We could allow to user to choose the language used to display the UI of discourse (it is deactivated for now to limit to English).
We could install the discourse-translator plugin (but it may has a cost).