I'm a daily DeepL User, the Windows App is convenient but nothing comparable was available for Linux, so I build my own.
jDeepL is a very simple GUI Application based on JavaFX, developed and tested on openSUSE Tumbleweed with KDE Plasma.
You can find the GPL v3 licensed Source Code here: https://dominik.wombacher.cc/~git/jDeepL/
It's in an early stage but covers the most important functionality: Translating the marked text by pressing
CTRL + C + C.
At least that's for me the most important one from the official Windows App ;)
I will probably improve it further within the next time, feel free to Contribute.
Further details can be found in the README: https://dominik.wombacher.cc/~git/jDeepL/about/