You can't. It only lets you pick one language, even though that definitely isn't how that feature is supposed to work. That's why it warns you that if you switch from Unspecified, you won't see a bunch of stuff. The flipside is if you aren't posting in Unspecified, people will not properly see what you're saying. You have to edit your posts to change the language to Unspecified or it's only showing up in my inbox and for people who also changed their language the way it says not to.