Looking for a fic I found several years ago, probably on livejournal, which had Sam joining the military and signing up to the Stargate (SG-1) program. At some point Daniel Jackson realises he has solid knowledge of a few ancient languages, and calls dibs on Sam as an assistant/protege. I don't think Dean was in the program with Sam (he definitely wasn't at the start, at least).

I can't be sure of anything more without risking conflating stories. It doesn't appear to have been offered as an answer to a fic request here before.