Just a slight correction, the guy in TS doesn't give you skellies then giants then centaurs. Its pretty random as far as I know. That's what you got, but another person will get a totally different order of quests. I kept getting giants while my friend kept getting crap like beetles and antelopes lol. Very frustrating (she kept laughing at me /sigh).
Second, there's a ton of language mastery out there.
Gnoll, giants, fairies, I'm afraid I don't know them all.
Fairies, the little stupid mobs in Peat Bog, will drop a fabric patch which, at level 31 (yes you read right, 31) will start the language quest. They are level 3 or 5 or something, so at level 31 if you want to do this quest you can kill level 3 mobs to do it. Silly eh? I assume there's higher level fairies that will also do this.
Giants - you kill giants in TS or wherever, and they will drop hill giant idols.
Gnolls drop the chiranite threat totems.
Orcs have a language quest.
Not sure what other ones have them, probably many of the races that have a mastery quest also have a language quest. Basically if it gabs at you in a language you don't understand, odds are there's a language quest for it :)
I know there's a bunch of others out there.
Now that they moved them to the drop table and not the chest table, you can get these language parts from even grey mobs.