Javier: are you sure that going through the "normal" steps of "International->Dictionaries" doesn't work for you?
I have had similar problems with dictionaries before, but I thought that that was because I heavily modded my Kindle. After a factory reset (including localization), dictionaries now work for me as well. So if you can afford it, could you check if your Kindle works with your dictionaries after performing a factory reset?
(As a "bonus", if you factory-reset while having locale packages installed, you'll get a boot-screen locale selection which you probably didn't see before
)