For a symptom like "FULL_EDITOR is showing some other string that what I've set in my language pack", I would also search your entire phpBB site folders (or at least /ext, in addition to /language) for anyone who might be (re)defining that string.
Not that they're "intentionally" doing it, necessarily, nor trying to affect the same display you're intending to affect. But it seems like I recall a case of either an older language pack for phpBB itself, or an extension for which an older language pack was being used, unintentionally caused string name collisions with later versions of phpBB.
Regardless whether that's specifically the case here, the more general point is that in addition to the place where you're supposed to be defining this string -- and currently are defining it -- also look for any other .php files which might be defining this same name, and perhaps in a place you may or may not have expected for it to have ever been defined.
The "find in files" of your preferred editor should help you easily search the entire set of site files, and rule out that possibility.