1. Users should choose their language in the profile
2. On mainpage language and currency should be placed above all else as banner (like steam with language selection... best placed around the "sign in" or "register" label)
3. thank you for doing this, but you presumed all people understand English from the beginning so keep this up and keep the page default English.
People have found their way to GOG, because they understand English and many of your catalogue is still English and will always be.
Be smart GOG Team.