I'm not a language expert, but what I've heard was that with the Creative Suite products it isn't easy to change language. In fact there's a form you can fill out to apply for a different language version:
If you go with a Creative Cloud subscription, all those problems go away. It's as easy as changing a CC preference to switch language versions.
I cannot recall for certain - I stick with the English UI at all times - but I think it matters whether you bought a strictly-Danish edition, or the multilingual edition. Also, I know that there are differences between UI language switching on CS6 versus earlier versions of the Creative Suite. Likewise, there are platform differences between Windows and Mac OS. However, I think that this one link covers all of your options.
As a Creative Cloud subscriber you can install (also additionally) any language without additional cost.
With the Creative Sutie versions the serial number limits to only one or some languages where this serial is compatibel. It was written on the package for which language it is compatible, in most cases it was restricted to only one language.
You could ask Adobe for a single platform and/or language changer per version. But this was limited to region. E.g. it was possible to change between EU languages EN, DE, FR, ES, … but it is not possible to change between US-EN to UK-EN or DE because the prices are very different in those regions. In Europe the software costs several times more then in the American continent,, like U.S.A. or Canada or States for Mexico.
But when a new version was released, it was not possible to make a language change as you could do a normal upgrade.