The Breen language was the language spoken by the Breen. To Human ears, it sounded like garbled electronic sounds. Universal translators needed to be adjusted in order to properly translate the syntax. (DS9: "Strange Bedfellows") Breen could be spoken by a person wearing a Breen refrigeration suit, though it was also possible for that same person to instead speak English while wearing such a uniform. (DS9: "What You Leave Behind", "Indiscretion") Speakers of Breen included Thots Gor and Pran as well as a holographic Breen. (DS9: "'Til Death Do Us Part", "Tacking Into the Wind", et al.; VOY: "Flesh and Blood")
In 2373, Michael Eddington sent a file info that was intercepted by Benjamin Sisko aboard the USS Defiant. It was a heterophonic, five-line song with an alternating tetrameter and pentameter structure, but other than the catchy tune, it did not mean anything. (DS9: "For the Uniform")
In 2375, although Cardassian leader Legate Damar was present when the Breen Confederacy allied itself with the Dominion during the Dominion War, all speech which was spoken by Thot Gor at the historic event wasn't understood by Damar, as he hadn't had a chance to adjust his universal translator yet. (DS9: "Strange Bedfellows")
Later that year, while Kira Nerys was disguised as a Breen and was wearing one of their uniforms, a Jem'Hadar guard who encountered her on Cardassia Prime remarked that he didn't understand something she said in Breen, though he was then immediately shot by her.