why do the aliens speak English?

The FAQ for Stargate: Atlantis admits the dirty secret of the "universal translator:"

Q: Why do the aliens speak English?
A: Practical reasons that come with television production. The time constraints of an hour-long episode mean that it would become a major hindrance to the story each week if the team had to spend the first 10 minutes of each episode learning to communicate with a new species.

I remember loving the "language" incidents in Star Trek -- the argument that Shakespeare is better in Klingon, the TNG episode with the language made out of metaphors (um, isn't that every language?), the DS9 episode in which the universal translator broke down -- good stuff. Of course, a better criticism of that world (which, I think, does not apply to Stargate) is why so many sentient species have achieved exactly the same level of technological development and why they are all genetically compatible. This was partially answered by the TNG episode that revealed the common genetic heritage of all sentient life (paging Dr. Rael), but we all know the real answer is television.