I updated my FW from 5.8.11 to 5.9.6.1 hoping that they added that feature meanwhile, but no.
Interesting that the Fire lineup can do that, sad that PW3 cannot.
EDIT: Figured how to make tomsem's workaround work after a call with support.
So, first of all, this is retarded, as you can only search a single dictionary at a time without having to manually switch them in settings.
I have a Norwegian to English dictionary I would like to search in.
In the "Language & Dictionaries" Settings tab, my kindle has "Language" set to English.
This means it'll use the dictionary that is set as default for English when using the search function (Which defaults to The New Oxford American Dictionary).
The dictionaries have a value for IN and OUT language. My Norwegian dictionary has nb-no for IN and en-us for OUT.
Now, this means that my correctly-marked nb-no>en-us dictionary will pop up under a separate entry without any language. -
https://i.imgur.com/FBIv0d0.png
So there is no way for me to select it when trying to make it the default dictionary for the English language -
https://i.imgur.com/LzhN5yc.png
The solution is simple and stupid. Incorrectly mark my Norwegian dictionary as en-us>en-us and set the default dictionary for the English language to my now-incorrectly-marked Norwegian dictionary.
This means that if I want to browse the English dictionary, or Norwegian Nynorsk dictionary instead of Norwegian Bokmål, I have to manually swap it to be the default dictionary.
And that still is just a workaround for not having a feature that the fire edition apparently has.