I have been thinking… and I believe we should go for the full package: enable automatic translation and let everyone post in their preferred language anywhere (except topics or areas specifically designated for a specific language). In theory, this is doable thanks to several giant’s shoulders:
Interface language
Discourse already allows user to select the language of the interface. We just have to enable this setting.
Multilingual plugin
The Multilingual plugin allows to identify the language of a post, the translation of content like categories and tags, and the ability for users to select the languages they want to see and filter out the rest.
Discourse Translator plugin
The Discourse Translator plugin offers automatic translation of posts by connecting to a translation engine. This allows users to post in their preferred language, and other users just need to click a 
button to get that text translated to their preferred language. These translations are cached for future readers, saving redundant calls to the translation engine.
LibreTranslate machine translation
LibreTranslate is a free and open source machine translation engine and API, that can be installed in your own server and feed Discourse Translator.
I’m familiar with all these pieces except LibreTranslate, which I have never user, even less installed. If anyone has advice for the simplest installation guide, I welcome all the help.
I plan to set this up over the weekend. Feedback welcome!