Well the Caitains are only fleshed out in TAS and Star Trek IV, them being a species and on the Federation Council in the 2280's is all we really know.
But they've seen a lot of mentions and building up in the books, some good characters and storylines that could easily be incorperated into future stories.
The language, I don't know when it was made, but it was part of some background material during the TAS days, an alphabet that looked like old Earth Cuniform, systematic scratches in groups representing letters and words.
That's Kzinti script - originating from Larry Niven's Known Space
book universe, where the "The Slaver Weapon" (adapted from the short story The Soft Weapon
) and the Kzinti were borrowed from. In Alan Dean Foster's expanded novelizations of the TAS episodes, Star Trek Logs
, the Kzinti and Caitians are distantly related, not unlike the Vulcans and Romulans.