I have been trying to automatically set language for my SSO users using the Accept-Language headers. As far as I can see this should be supported by Discourse.
I enable the two options below:
Then with Chrome, I use an extension to set the Accept-Language:
Unfortunately, my UI stays in English. I tried different formats for accept-language (fr, Accept-Language: fr-CH, fr;q=0.9, en;q=0.8, de;q=0.7, *;q=0.5…) with no luck…