A dialect was a variant of an existing language.
The Klingon language featured eighty poly-guttural dialects. In 2151, the universal translator of Enterprise could not lock onto Klaang's dialect. (ENT: "Broken Bow") By 2154, the UT of the Enterprise had been programmed with seven Klingon dialects. (ENT: "The Augments")
The frequency distortions created by a symbiotic lifeform reminded Hoshi Sato of the phonetic patterns in some Andorian dialects. (ENT: "Vox Sola")
The Xindi-Insectoid language had sixty-seven dialects. (ENT: "The Council")
The Romulan language had three dialects. As of 2258 of the alternate reality, Nyota Uhura was able to speak all three dialects. Upon learning this, Christopher Pike asked her to relieve Lieutenant Hawkins at communications, as he was unable to tell Romulan from Vulcan. (Star Trek)
In 2267, Captain Kirk told Spock that the baby language used towards Leonard James Akaar was an "obscure Earth dialect". (TOS: "Friday's Child")
The Talaxian language had an "Old Tongue" dialect, used in the 15th century when the Talaxians referred to themselves as the Talax'ilzay. As a result of their encounters with the Vaadwaur, "vaadwaur" had come to mean foolish in Old Tongue. (VOY: "Dragon's Teeth")