Did Tolkien ever say why Gondolin was so called-called by its Sindarin name, instead of the Quenya: Ondolindë? Thingol forbade his people from using Quenya or responding to people who used it, as a response to the actions largely due to the FeanoriansFëanorians, but the Elves of Gondolin, part of whom where Sindar, were led by Turgon, a Noldo, and obviously had no contact with the rest of Beleriand. If I remember the timing correctly, the move by Turgon's people from Nevrast to Gondolin happened after the ban on Quenya, but I find it peculiar that Turgon didn't maintain the Quenya name of his kingdom. Clearly history is written by the dominant group etc etc, and almost everyone else was speaking Sindarin in the outside world, but Thingol wasn't around (or around for long) to enforce the name change once Gondolin became known to the rest of Beleriand.
It is said that Turgon appointed its name to be Ondolindë in the speech of the Elves of Valinor, the Rock of the Music of Water, for there were fountains upon the hill; but in the Sindarin tongue the name was changed, and it became Gondolin, the Hidden Rock.
Aside from everything having multiple names in at least two languages, is there an in universe reason, or something from a letter explaining why the Sindarin name stuck?