Eastern Europe has a lot of vaguely similar languages, so assuming he was Russian, while hardly offensive, was probably a decent leap.
Just looking quickly at Google Translate, "da" is also "yes" in Ukrainian, Serbian, Bulgarian, and possibly others, and is also similar to the widely used "ja." Albanian "yes" is different, but it's possibly colloquial or casual.
Still not rude to offer your thanks, though I might stick to English from now on.