In the Stargate SG-1 episode "Crystal Skull," the alien does not speak in ancient Mayan words. The alien communicates telepathically with the characters in English.
"Uy ah ual ing ual ing wetail." (The enemy of my enemy is my friend)
If you are asking about what language is 'used' in the television series Stargate, then the answer is English. But if you are asking about what language is used 'on' the stargate device (the words written on it and the chevrons) then the answer is the Ancient's language.
Chinese
The Celtic language was spoken by the ancients in the regon, but the language spoken in Britain is not known. It was not recorded.
No. The Ancient Hebrews spoke Hebrew.
There is no record of the language(s) spoken by the ancient Beothuk people.
No one alive today knows how ancient Egyptian was spoken.
Latin
Indo-Iranian
Uxmal is an ancient Mayan city in Yukatán, the language spoken there was the Mayan language.
Latin.
Latin.
The Romans were Latins. Therefore, they spoke Latin.