The IAU is responsible for naming conventions and certifying proposed names. There are certain themes for different classes of objects- all new features on Venus must be named after women, the moons of Uranus are named after Shakespeare characters, kuiper belt objects are named after deities, etc. It is usually the discoverer who chooses the name to be certified by the IAU.
Astronomical objects known to humanity since before the IAU developed this procedure, including many surface features of the front of the Moon, often have historical names that defy their later convention. Now, maria on the Moon are typically given Latin names of abstract, positive nouns, e.g. Mare Tranquilitatis, "Sea of Tranquility." Craters are named after astronomers. Large Martian surface features have Latin names, e.g. Olympus Mons, "Mount Olympus," also consistent with the earliest observations of Mars occurring when Latin was the lingua franca of science.