Academic Job Mobility in the European Context In the United States it is not uncommon to apply to academic jobs all over the country. Personally, I do not know a single person who limited his or her search to just one state (i.e. Massachusetts). That being said, the language of instruction and the key aspects of academic culture remain the same coast to coast. 

In comparison, what is the situation like in Europe, particularly Scandinavia and Switzerland?

For example, given that the population of Denmark is comparable in size to that of Massachusetts, how does this affect the academic job market? Is it standard practice for PhDs &amp; lecturers/postdocs located in Denmark to search for their first career placement across Europe [and beyond]? Or do they search for employment primarily on the national academic job market? How do the national differences in language/academic culture fit into this equation? 

Background: I’m considering PhD/Academic Career in Europe. Ideally, I would like to learn the local language and assimilate as much as possible during the PhD. Given this long-term effort, I would prefer to continue on in the same country following graduation.

I’m especially interested in hearing from those with experience in the social sciences and humanities (working or studying in Denmark, Finland, Norway, Sweden, or Switzerland/Austria). However, please consider contributing even if you do not fit this particular set of criteria. My own experience is in Cult/Soc Anthropology (USA).
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.