Dick knows a lot of languages, both from Batman's training and from trips with the Haly Circus in which he was able to travel all over the world. For most of the languages he already had a basic knowledge precisely because of the trips with the circus. Dick knows many more languages than Bruce and his brothers do.
Dick knows Romani (native tongue), English, Italian, French, German, Spanish and Portuguese, Chinese and Japanese (and much like them), Russian, Arabic (improved later thanks to Damian), Latin, sign language and Morse code, and also speaks alien languages such as Kryptonian (Clark and Kara), Atlantean (Arthur and Garth), Amazonian (a.k.a. ancient and modern Greek thanks to Diana and Donna) (not an alien language but details), Martian (Martian Manhunter and Miss Martian), Tamaranian (Starfire), Thanagarian (Shayera Hol), he also understands speedster speech well (Barry but especially Wally) and he also speaks many other languages that he uses more rarely but still knows very well.
There is a reason after all that Nightwing is considered the translator of the hero community.