In Canada, we often wish to give the user their choice of language interface; this needs to be a toggle, so the user can switch from one interface to another without losing where they are in the session.
Then, the user must be able to create posts in any language through their chosen language interface.
Finally, the user must be able to stipulate which language posts are delivered within their chosen language interface:
a) all posts in any single language; or
b) posts in two (or more, or all) languages, with the results combined into one display (chronological, or by tag or thread, or ... ...)
There would be much Canadian adoption of such a plug-in. I don't code; but I am willing to test and comment on other folks' installed alpha or beta possibilities.