Hmmmm....

6. Why does every culture SG-1 encounters speak English?
Practical reasons that come with television production. The time contraints of an hour-long episode mean that it would become a major hinderance to the story each week if SG-1 had to spend the first 10 minutes of each episode learning to communicate with a new species.

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Taken from the Stargate SG-1 FAQ... you know that is a answer that actually works for me.