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?
How is this relevant?
You can now use telegram with any matrix compatible client you like
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.)