Your 'You can only set a default dictionary, if you have multiple dictionaries for the same language installed' clarified things a lot.
I have the two dictionaries I want to use loaded and shown in Home - Dictionaries.
Home-Settings-Dictionaries shows both the English and the Spanish dictionaries set as Default, and I now understand this means 'Default for that language'. (Checked this and your point above by adding an American english dictionary - 'Change Default' option is shown!)
I've edited the metadata for a few Spanish books and checked that they are formatted as *.mobi, and that Language is set to Spanish, using Calibre.
When I try to look up a Spanish word my Kindle chooses the English dictionary - obviously without much success!
Am I doing something wrong or has my Kindle got a problem?
Can you please help me sort it out?
