I don't know if it has been posted before, but I haven't found it so I'll just share:
It is not possible to select the primary dictionary for any language other than english.
But... my kindle seems to recognize when a prc book has another language setting and chooses some random dictionary with the same language as input for instant lookup.
so all I needed to do was to change the input language of all my german books to german, my spanish to spanish... and so on.
Mobipocket creator doesn't do that but some great guy wrote some pearl scripts that'll do it:
https://dev.mobileread.com/trac/mobiperl
So I put all my german books in a folder and since I'm a lousy windows user entered in the command:
for /R (your folder) %g in (*.prc) do perl mobi2mobi --outfile "%g" --language de-de "%g"
I'm not sure about the language option -hex digits didn't work for me... spanish worked by just entering "es" (without quotation marks)
then I used the mobipatch tool from above to set all my other german dictionaries to romanian except for the german wikipedia
http://www.axelschaefermdb.de/service/wikipedia.html
(märz 2005 is the latest that will work)
doesn't know any inflections but is still neat...
and now I'm gonna do the same thing for my spanish books
