Nope, you are right. The lang files are part of the site still and should be deployed to the production environment. Keeping them to a separate VPP as part of the installation would make more sense I agree but I guess EPiServer wanted to make it simple to override some language settings and therefore chose to keep it at the site level.
But yes, if you see any "Missing" texts, make sure that all lang files are in that folder :)
We always have to include the default EPiServer lang files whenever we deploy our site. If we don't include them we see "missing" everywhere in the UI. I just wanted to check with you if this is "the way to do it"? It would have made more sense if the EPi lang files were placed in the EPi-installation folder (under Program Files) instead. Since it is always possible to override strings in the local lang folder if needed... Or are we doing something wrong?