There isn't an Irken language. There's irken "writing". The reason that Irkens and other aliens speak english has yet to be solved. Perhaps early humans might have discovered a primative english that branched off into many other languages. Primative aliens might have used this primative english as well. That being said, english (or whatever other language you watch Invader Zim in) may very well be the universal language of the universe, so that other alien species can communicate more easily.
The reason the Foodcourtia signs are in english is because if english is the universal language, they want to attract as many custermers (sp?) as possible.
It might be that it is all traslated into english, because if you watch when Dib is talking to the Tallest at the end of the episode "The Frycook What Came From All That Space", he asks them questions that they never reply (because they dont understand?) and they say his head is big; Dib normally says "My head's not big!" when this is said. maybe they cant understand english, and he cant understand Irken, and only Zim and Gir can speak both because it is vital for their mission to understand english?
"There plastic, there spastic, there fantastic."
