But the moment you start using existing language in a new context, you get into issues. You've got words and whole concepts missing that you have to add to something that's more or less complete.
Dothraki needs horse-related words. You need to be able to express certain social concepts that are unique to this fantasy culture.
So you set up to build this words and phrases. You derive phonotactics of a real language and patch it with your stuff. Now you've got new language based on an existing one. So you didn't preserve anything. And if I were a user of the originl lang, I'd be upset you appropriated and twisted it.