Telegram's problem with persian font
Solution 1:
There is similar topic in askubuntu question about telegram and its bad support for Persian language fonts.
A good news is that Telegram on newer release add default support to Persian with adding Vazir font family.
For you I suggest to remove telegram-desktop
package if install with Ubuntu repository and download the latest one from official website to use latest update of telegram.
apt purge telegram-desktop
If you prefer more info on installing telegram in this method please read my answer on Iranian UBUNTU Community.