(covers information from several alternate timelines)
Chancellor was a title used both by nations as well as planetary governments to refer to their head of government.
In an alternate timeline, Adolf Hitler was the chancellor and Führer of Nazi Germany in 1944. (ENT: "Storm Front, Part II")
Since the mid-21st century, the government of the Klingon Empire was headed by a chancellor. (TNG: "Rightful Heir")
In 2151, Kalev was the chancellor of the Coridanite government. (ENT: "Shadows of P'Jem") In the alternate reality, Magustav Pachangara was elected chancellor of Coridan in 2259. (Star Trek Into Darkness)
By 2152, a nation-state on a planet was led by Chancellor Kultarey. (ENT: "The Communicator")
The same year, Chancellor Trellit was a leader of the Torothans. (ENT: "Desert Crossing")
The Andorian Empire was also led by a chancellor. In 2154, the Andorian chancellor contacted the Aenar to arrange a meeting with Jonathan Archer. (ENT: "The Aenar")
In 2367, Avel Durken was chancellor of Malcor III. The position of vice-chancellor also existed. (TNG: "First Contact")
In 2368, Alrik served as chancellor of Valt Minor. (TNG: "The Perfect Mate")
In 2374, Gaumen held the office of chancellor of the B'omar Sovereignty. (VOY: "The Raven")
In 2380, Commander Jack Ransom of the USS Cerritos led an away team that met with the Chancellor of the Galardonian High Council. (LD: "Second Contact")