Hi Arun,
Under ICF node /default_host/sap/bc/ui2/start_up ---> Error Pages ---> Configuration, you can set the default language, it will effect all users.
I guess there is not a way to set user specific language for Fiori if we use the default fiori system logon page(as I explained in the previous reply). You can manually set it on the logon page or in the URL.
In other cases like Single Sign-On, I think it would follow the mechanism as you mentioned at the very beginning. (Not for sure since I didn't test it)
Best Regards,
Jessie