Catalina is in Turkish but some apps are in Russian I have a MBA and the language is in Turkish. However, some apps are in Russian. The only Russian-related thing on my system is a keyboard layout. Aside from that, env in terminal shows nothing Russian related. The latest app I came across having Russian in Aegisub. How can I stop this behaviour?
If your app includes an English localization, you can get this to display by going to System Preferences > Language and Region and making sure that English is the second item on the list of Preferred Languages, right after Turkish.  Input Sources preferences are irrelevant for your problem.