use it whenever you like. but you can leave it out when its omission does not bring confusion. here the speaker suggests a movement, we can easily assume that they are talking about the country, and not its population or language (which is not of course, they speak French or Flemish). well this is not the best example because populations and languages are normally preceded by người and tiếng. let's use Đức, Mỹ, or Nhật instead, respectively Germany, USA and Japan: they can also be a person's given name. when I say "tôi thích Đức", I need to add nước before the name otherwise I could be understood to like someone called Đức.