Learn Japanese To Survive! Hiragana Battle comes with an html version that is DRM free and that appears to be platform agnostic. You just need a browser that can run html (likely html5) and a way to give inputs to the game. To launch the html version, go to the folder /HiraganaBattle/www/ and open the file index.html there in your favorite browser.
Tested in an Ubuntu 20.04 virtual machine that never had Steam installed.
Also, according to an user from protondb, if you use Firefox, you need to toggle a setting in it for the game to work. For that, put about:config in the link field and open it, search for "privacy.file_unique_origin" and set it to "false". Then it should work without issues after that.