Because getting "common language and culture" is not neutral when we reside in a world that basically this works with american culture being the global culture. Honestly imo this is a kind of perspective that only an american could have, and being ignorant about what it actually entails. You do not need everybody else to adopt a certain language and way of living to get to understand each other and have peace.
Moreover as other examples here given, geopolitical tensions very often appear between neighbours who have a very similar culture, so sharing cultural similarities does not seem to lead to much of peace in the world.