The Rodian language was 'created' by Star Wars sound designer Ben Burtt. The language is, depending on whom you believe, either Quechua or heavily based on the sounds of Quechua.
The Star Wars website says "Greedo spoke Quechua, a language indigenous to the Andean regions of South America".
I have heard an interview with Ben Burtt, however, in which he says that he spliced Quechua speech to come up with the final sounds heard in A New Hope.
More confusingly, the same official Star Wars site also lists Greedo's lines in a discussion of Huttese. Bounty hunters and smugglers would both be likely to know Huttese as a lingua franca of the gray market.
Ultimately, the real truth is that it's a work of fiction, and any canon of equivalent complexity has continuity problems that can be found by casuist fans.
SOURCES:
Greedo's speech is Huttese: http://www.starwars.com/eu/feature/20010808/indexp3.html
Greedo's speech is Rodian, which is Quechua: http://www.starwars.com/databank/character/greedo/bts.html