The telegram app has a very nice interface, but I want to use a self hosted xmpp chat server.
Is there maybe a fork of telegram that makes it work with a self hosted xmpp server? I would imagine that this is possible.
If not, is there anything that at least gets close to how nice telegram UI is?
mautrix/telegram is a bridge between Matrix and Telegram. It mostly lets users of Matrix contact their friends who use Telegram. It is not a fork of Telegram and has nothing to do with the Telegram interface. (Note: OP wanted to use the Telegram client with a non-Telegram server. If you know of a Matrix client which looks and feels like the Telegram client, thats what theyre after.)