Hi Matías
As Kirk says, you will need to experiment by captioning files with various different Default IPTC encodings set in the IPTC/XMP preferences of Photo Mechanic
I can't offer much help with a specific one for you, but I had problems with captions containing Irish words with accents (much smaller problem than in Spanish, only applies to vowels which can have an acute, like French aigu accent - á é í ó ú.
I found that when captioning with "Write IPTC as Unicode" set, nearly all picture-desks could read the captions - but when images were uploaded to Photoshelter the accents would come out as a? e? i? o? u?, which certainly makes it difficult to search for them afterwards!!! This is because Photoshelter can't yet read 8-bit (it will happen at some time in the future I'm told)
The one that works for me is Mac Roman - not Latin, that was 10 times worse!!
Using XMP is fine, except a lot of places I send images to - newspapers - can't read XMP, their system file browsers are too old, they have to have IPTC
Keep experimenting - you might find one that works for you
Neil