Systems languages tend to operate very close to the hardware, with
little abstraction. Applications languages tend to be more abstract and
operate at a higher level. Cython straddles those two worlds, so it is
hard to classify as a systems or applications language, but I think I
would probably class it as more of an applications language.
Regarding memory management:
I think that you can run most Python programs in Cython with no change
to the source code. You may not get a lot of benefit, but you will get
some. But if you re-write the Python code in Cython, you will get even
more speed benefit.
I believe that you can run Cython generated code on Android. It looks
like DearPythonGUI does not support Android, so I don’t think you will
be able to use it on Android with or without Cython.
I don’t know what you consider a “great complete game engines”, but you
should be able to use it with Kivy.
Disclaimer: I have never used Cython. All my answers came from googling.