I'm not quite sure how that works on Facebook and Twitter, but yeah IPS doesn't translate stuff people write. If you're using Google Chrome to "translate" the site for you, that's not making any changes to the way other people see what's actually written. So it's not like it's "going back to the original language". In reality, it never changed so opening a new webpage might ask you if you wish to translate it as well.
That being said, I think it might make sense to install language packs. It'd make stuff like navigating through the settings easier for some people.