UMTLibInherited class probably not needed

[Miepee]

https://github.com/VladiStep/UndertaleRusInstaller_GUI/blob/main/UndertaleRusInstallerGUI/UTMTLibInherited.cs

This file is AFAIK unnecessary.

[VladiStep]

It is necessary for code import, and also it contains message box overloads/methods.

[Miepee]

but you're not providing an actual .csx scripts. you have the whole install written as your code, which means you should be free to not make it inheriting from IScriptInterface, and instead just split the functionality.
i.e. into a code import class and a message box helper class