We're a paid SaaS so I don't think it's as much of an annoyance to be asked once per browser.
It's just a separate page you hit after signing in if we don't have your preference. You click on the language you want and you are redirected to the dashboard.
The option to change it still exists in the footer.
Okay, must say that does sound like a very acceptable flow. You'll have the user logged in at that point so can store it and never need to ask again. Was expecting you built this for regular websites, my bad!
It's just a separate page you hit after signing in if we don't have your preference. You click on the language you want and you are redirected to the dashboard.
The option to change it still exists in the footer.