We've heard the shiny explanations before...
"They're not really speaking English - they're speaking their own language to each other."
"They're not really speaking English - someone is translating."
"The Stargate imprints extra languages, etc. etc." (I still do like this one, though.)
It seems that in season ten, English is definitely the dominant language of the Milky Way.
-The Ancient riddles Daniel translated into English actually rhymed in English.
-Vala assumed that the Ancient words for directional right and correct would match in Ancient as they do in English.
-Vala's father was familiar enough with English that he was able to start setting up scams on Earth almost immediately. Also, he was familiar with Earth phrases like "Is this thing on?" before coming through the gate.
It seems to me that English really *is* the language of choice across the universe.