CMS folder is our library, so it's easy for us to couple it to `JLanguage` and `JText`.  So I'd go with using the old `JLIB_` prefix and updating the strings.

Unrelated, but I'm pretty sure we can clean up the `en-GB.ini` and `en-GB.lib_joomla.ini` files with all the language decoupling that did take place, probably clear out quite a few lines in there.