I tried, and this fixes the problem
Very illogical to have Steam override ingame settings

No, I haven't tried setting the language in Steam, I didn't know that was an option. I'll try.

Yes, it doesn't matter if I change in the bottom right on the main page, or from the settings. Both times it will change to the system language when starting it on the other machine, it will stay correct if I don't change machine I play on.

I have 2 factorio installs via Steam on my work laptop and my home pc. My work laptop has a Dutch version of Windows, and my home pc an English version. If I set my language to English (on both), it will set the language back to Dutch automatically if I started Factorio on the Dutch Windows version ...