There is very little known about the Cardassian language, other than its proper nouns (see Cardassians and Cardassia for names of people and places respectively).
Writing system
Cardassian writing was composed of slender rectangular letters usually widening at the top. Lines of text were written both horizontally and vertically, radiating out from an elaborate circle motif. Lines of text also incorporated dots. (TNG: "Chain of Command, Part II"; DS9: "Tribunal", "Things Past")
Keiko O'Brien's classroom aboard Deep Space 9 featured a chart with the Human, Ferengi, Bajoran, and Cardassian alphabet. (DS9: "The Nagus", "In the Hands of the Prophets")
Military and government
"Gul" was a rank that was roughly analogous to the Federation rank of captain; "glinn" was a rank below Gul. "Legate" was a rank above gul (see Cardassian ranks). (TNG: "The Wounded")
The Detapa Council was the highest civilian governmental body, although the meaning of the word "Detapa" is unclear. (DS9: "The Way of the Warrior")
Miscellaneous
"Odo'ital" was Cardassian for "nothing". Odo was called odo'ital when he was studied at the Bajoran Institute of Science. The Bajorans working at the institute intended to label Odo as "unknown sample", but all specimens had to be labelled in Cardassian, and Odo stated that the Bajorans mistranslated the term by incorrectly using "odo'ital", which literally meant "nothing". (DS9: "Heart of Stone")
"Yamok" sauce was a common Cardassian condiment. (DS9: "Progress")
"Nor" carried connotations of either "station" or "base"; Terok Nor and Empok Nor both had this word in their names. (DS9: "Cardassians", "Empok Nor")