Does an Int lower than 9 impact the Read Language thief skill?
I’m kind of amused by the idea that a character would be just as likely to make sense of a dead tongue as they would their native language, but I’m not entirely certain it’s intended to work that way.
As written, a thief with INT 8 or less would be unable to read anything unless he made his Read Languages check.
If I were judging the PC, I'd allow him to automatically succeed on reading his native language, though, just to keep things from being too silly.
I’m not sure there are enough penalties for a low Intelligence, though. As written, there is no proficiency penalty applied to starting proficiencies.
Perhaps I’m just cruel, but I might make such a thief spend their general proficiency on Language if they didn’t want to be unable to write or have to roll each time they were called upon to read. 
I would handle it as some sort of gift/reading disability where sometimes words and sentences make no sense no matter what language they were written in.