I want to assign a particular file to open in a specific non-default application.

For instance, I want to open all text files in a konsole in vim and I have a .desktop file for that in my .local/share/applications

But there is a text file containing some non-english characters and I want to open it in Kate.

Can this be done? How?

Unfortunately i'm not sure if this is possible. The only way I can think of doing it is manipulating the "magic" which KDE (among others) uses to determine the mimetype of files. Each mimetype could then have different associated applications.

I would suggest checking the documentation on freedesktop.org, relating to file associations and mime types.