2 Answers
+ 6
What is the "hardest part"? I think GUI is the hardest to do :) I mean what could be too difficult for c#?
But making GUI in Python is really easy too, you just need the right libraries. tkinter is built into standard python, and you can even use it for graphics (canvas) or draw plots with matplotlib. There are also other libraries for python: pyqt5, wxpython, kivy, pysimplegui...
Believe me, you are much better off implementing everything in the same language, rather than parts in python and parts in C#.
+ 2
Thanks @Tibor Santa