Simple: give me a popup the first time it is not clear what language I want content in. Put a don't ask me ever again tickbox on this popup.

I will click No, keep content language and tick don't ask me ever again and be super happy.

If you really want me to like you, put a tickbox in the settings so I can save this as default into my other browsers (that don't use cookies / don't save data)

Thank you !