I've been studying at Leiden University (which also has a Liberal Arts course) for almost two years now. In my own experience, and from what I've seen of others' experiences, the international students do tend to congregate together. This isn't actually a bad thing in my opinion; we're a group of people with similar motivations in life, the same enthusiasm for diversity, and in the same situation, living in a foreign country and wanting to explore, so we have a lot in common and enjoy each other's company. It's great to make friends with people around the world!
Having said that, I also have very good Dutch friends and it's easy to talk English in social situations. Having said that, I think it's hard for any outsider to become part of an existing friendship group. This is true in any country. So my Dutch friends are friends as individuals, rather than a big group that I've joined. They have their own separate social lives, just as I do.
Also, I've found that the extra-curricular activities are largely in Dutch (which makes sense, and isn't fair to change), so it's difficult to get involved in societies and fraternities/sororities, that kind of thing. However, the international students have their own very diverse and active set of societies.