Okay so here is what I did (obviously I have nothing better to do
If you select a word with the cursor in your kindle- the translation pops up. Then if you press enter you get to the dictionary. And now if I select a word within my dictionary- the definition from another dictionary pops up.
So you can use, let's say an english-english dictionary and from within the english-english dictionary go to an english-german dictionary.
This somehow works because the metadata of the dictionary translation language is stored separately from the ordinary book's language.
To do this you need to change the language of your primary dictionary to some random language. For this you can use the mobi2mobi perl script (for which a GUI exists as well named Mobi2Mobi_GUI_VB).
After you changed the book language to, let's say french, you take your second dictionary and change the translation metadata. For this you can either use the mobipatch file posted in this thread- or you can directly change the hex-code to the same language (french in my case).
BTW the restriction that only english dictionaries can be used as primary dictionaries obviously doesn't apply anymore, probably since the last fw update.
Usually I like an english english explanation better, but sometimes I'd like to know the german word.
unfortunately I can't manipulate the Oxford dictionary that comes with the kindle (which I like very much). If anybody knows how to do that please tell me