In the Clone Wars S03E04, Greedo speaks
English Galactic Basic/Common. However, in "A New Hope", Greedo speaks... well, whatever he speaks, it's not English. What language is it, and also, does he manage to un-learn English in the next 20 years?
Greedo is probably speaking Huttese in A New Hope. I say probably because there doesn't seem to be a canonical source that explicitly states the language in use.
Supporting, but not conclusive evidence for this:
- several of Greedo's lines appear on the "Speaking Huttese" page at Wookieepedia
- Wookieepedia also lists Huttese as one of the primary languages for Greedo's people, the Rodians. It cites the Galactic Phrase and Travel Guide for this information, which is a licensed work but not canon -- yet apparently explicitly states that Greedo's part of this conversation is Huttese.
- he is currently in employ of a Hutt (Jabba), who is never seen speaking anything else.
- other collaboratively-edited sites (such as IMDB) typically state it as Huttese in their quotes.
- the script simply lists "a foreign tongue", but this is one of only a few occurrences of non-English dialogue.
- the novelisation makes out that they are both speaking the same language, or makes no mention of Greedo speaking something different.
- Rodians didn't even have a name until after the film came out. It first appeared in an RPG supplement. Like a lot of other things to do with Star Wars, people have built up much around it that isn't necessarily supported by the original sources.
Anyway... That he chose to speak this language to Han can not be taken as evidence that he didn't know Galactic Basic at this time -- he knew Han would be able to understand, and was able to understand Han's responses to him. Particularly the blaster bolt.
Huttese is the second most common language in the galaxy. Choosing to address Han in this language has no disadvantage.
In his appearance in the Clone Wars TV series he is talking to people who are somewhat more removed from the criminal spheres in which knowing Huttese is expected. Talking to Chairman Papanoida in Basic is necessary here.
Plus, there's really no need to make us read in a cartoon, is there?