As far as I can tell, the answer would be left up to Christopher Tolkien as Forion mentioned. Tel'Mithrim elvish is indeed derived from Tolkien elvish, just some things are changed here and there. Mostly simplifications and new words were added. I know this probably isn't the answer you're looking for and it's quite possible that it's not under copyright and i'm completely wrong. Now that I think about it as far as I recall it's not exactly easy to copyright a word, much less a language.... *thinks* Either way I know of a few language generators and the like if you'd wanna use it for your own version of elvish. I actually have quite an extensive language i threw together with a generator for a class once if you'd like to take a look. e-mail me above if so.
-Therfalas' Player