Superman and Supergirl may be the only Kryptonians, but they arent the only aliens.
Whose to say there isn’t some alien around who has knowledge of a lot of galactic languages. It wouldn’t be hard for a Green Lantern to contact someone who knows that language to verify it if need be.
Luther pulled a fire breathing giant Stitch out his ass and it didn’t shock anyone. So someone being around who can decipher that language is not a big stretch.
We just watched a movie where not only did a man create pocket dimensions that housed aliens from all over the galaxy but another guy showed up and hacked them immediately and the big plothole for yall is how was a language translated.