Naoko Takeuchi didn't pull the Silver Millennium character names out of a hat. She raided Greek mythology for them.
Selene is the Greek name for the goddess Luna.
Selene
(Encyclopædia Britannica Article)
In Greek religion, the personification of the moon as a goddess. She was worshiped at the new and full moons. Her parents were the Titans Hyperion and Theia; her brother was Helios, the sun god (sometimes called her father); her sister Eos (Dawn); and her husband Zeus. She is most identified with Endymion, whom she loved and whom Zeus cast into eternal sleep in a cave.
"Serena" is a name chosen by DIC, and it was a deliberate play on her normal personality vs. her Neo-Queen Serenity persona. It comes from Latin, but it means "serene, calm". It has nothing to do with "Selene". Her name in Japanese was Usagi (Bunny), a play on the Japanese idea that the patterns on the moon look like a rabbit pounding out mochi to them. In Europe, of course, they thought the patterns looked like a man's face.
Strangely enough, the Rabbit in the Moon is a myth that was also common with many of the ancient Native American city-state cultures between Mexico and Panama.