I finally managed to accidentally click on some chinese scribbling that presented me with language selection. So I'm now able to use the phone and hopefully try to recover the DRM keys.

Thank you all for the help! For now it remains a mystery to me why I couldn't flash successfully with linux, but that it was at the end a success is most important. And a word of encouragement to others with my predicatement, it was much less a laborous process to do it under Windows than I was afraid of. Unlike many guides said, I did not have to install any drivers etc. Just download FlashTool and feed the image to it.