Academic Job Mobility in the European Context
From the perspective of the Netherlands and the social sciences I can say that international mobility is certainly common, but staying in your own country is even more common. I would say a small majority does indeed limit it's search to the Netherlands. 

This has consequences for the job market. Since the Netherlands is a small country the (sub-(sub-))discipline typically consist of a small number of people who will regularly meet. So if a position is open, then most candidates are known to the people who are hiring. So networks are important, and you typically get introduced to those networks by your advisor during your PhD. So doing a PhD in the Netherlands would set you up well for a subsequent career in the Netherlands. It is certainly possible to enter the Dutch market later and it happens regularly, but it is easier if you started as a PhD student in the Netherlands.

An important part of the reason for limiting yourself to your own country is probably the language barrier. You may be able to do a Postdoc with just English. However, when your job includes teaching, then you will eventually be expected to teach in the local language. Similarly, if your job requires you to participate in the self-government of the university, you will be disadvantages if you don't speak the local language. Moreover, your social live will be much easier when you speak the local language. Many languages in Europe are fairly similar, e.g. Danish, Swedish, and Norwegian, or Dutch and German, or Spanish and Italian. However, it is still a non-trivial investment to become sufficiently fluent to teach or negotiate in the other language. 

I am now living and working in Germany, and my impression is that mobility is less common here than in the Netherlands.