I do not know if you have seen it, but there is a Python GDNative library for Godot that adds Python support. It looks like it only supports desktop platforms on export, and the process is a little strange, but might be something to look at if you are interested in using Python with Godot.
As far language popularity and applicability, learning C# would be the way to go, as it is not a niche language (not like GDScript at any rate) and C# can easily be used outside of game development. The only thing I would note is that C# in Godot isn't used as often as GDScript, so you may need to convert GDScript code to C# when going through some Godot tutorials and learning resources.
Having used Python and GDScript extensively, I would say that GDScript is very similar to Python. A lot of what you learn programming in GDScript would be usable with Python, though there are small adjustments and differences.
Going with C# cannot hurt because there is a lot of options for C# developers, both in game development and outside of game development, but it different than Python and GDScript. I'd say is isn't quite as beginner friendly, but it certainly isn't a hard language to learn (in my opinion).
What you will learn programming, especially in the beginning, will be applicable to any programing language. I've programmed in all three languages (and more) and I'd say every programming language has its ups and downs, so I'd go with whatever works best for you. :smile: