Excellent point, and of course the computers have tranlations such as that of Narim the Tollanan. But some aliens have their own languages that are not translated on occasion, i.e. the Goa'uld and the Abydons.
But SG1 is never amazed how all the aliens can pretty much communicate with them in standard English, even if they do not relate to idioms and slang. Why wouldn't at least one planet have Russian or Chinese as their native language?!