From Stargate Wiki
English is a language spoken on Earth. It is the primary language of many countries on Earth including the United States of America, Canada, the United Kingdom and many more. The language is based on Northern European languages but also has roots in Latin and Greek as well as several others. The language is also spoken throughout the Milky Way and Pegasus galaxies.
Behind the scenes
The prevalence of English throughout the franchise's multiple galaxies is a conceit of production and for storytelling expediency.