i think the gems spoke russian.
you can see here that there was a gem station right smack dab in the middle of russia, with a huge crater-like sea surrounding it. i think most of the gem activity happened here, and it caused the earth to cave in around it, but of course they never finished because of the rebellion. while they were here, they adapted the language.
you can see here that the inside of peridot’s drill has russian written on the buttons.
and others have commented that centi’s writing looks like russian cursive.
so it is possible we could decipher gem writings from AFTER the earth was colonized, but everything before that is written in a non-earth language. i wonder if we’ll ever learn anything from that language!