I found an article telling how to do it.
Download the other language by switching to that language in the launcher's Settings menu (upper right)
Once done downloading, put the language back to English in your launcher, then start the game.
At the command prompt in game, type:
/script SetCVar("language.2", "fr")
Now the interface is in English, but everyone speaks French.
note: I did this with de for German, I'm making the assumption that fr is French.