Maybe I'm missing something, and also last time I worked with multilingual sites on Kentico was on v6, but that's exactly what the default culture is for.
I'm not sure how exactly Kentico figures the current user language, but I think it looks at the browser's culture, which depends on the browser and OS, is set in different ways.
I remember seeing a thread that lists the exact steps by which Kentico is figuring which language to show.
What exactly is your problem?