It's possible to add a different dictionary. I just now added one to my new Kindle. It has to be in prc or mobi format and de-DRM'd (I assume that's what you have).
I think you can only have one dictionary at a time - i.e. the one set to default. This user added an English-German dictionary:
https://www.mobileread.com/forums/sho...&postcount=288
You just add the dictionary to the other books and then you can set it to default.