There is a similar request here: https://meta.discourse.org/t/setting-user-locale-via-sso/35528
Would it work for you to have the signed-in user’s locale default to the locale from their HTTP headers, but also give them the option to set their locale for Discourse in their user preferences?
If so, it looks like this can be easily done. I have a working prototype for this now. The only downside that I can see for doing it this way is that until the user sets their language preferences with Discourse, the emails they get from the system will be in the forum’s default language.
Another option is to change how Discourse single-sign-on works so that it accepts a locale parameter and uses that to create the Discourse user from the SSO parameters.