I have several English and Spanish dictionaries on my Kindle(s). I was using the Larousse Spanish Dictionary to read Spanish until someone gave a copy of the Diccionario de la Lengua Espaņola (the official version, which comes on the K4). When I was using the Larousse, whenever I want to change to another Spanish dictionary I was able to do so. But after I started using the Diccionario de la Lengua Espaņola, I can't change to the Larousse or any other Spanish dictionary.
The same was happening to me with the Spanish-English Pocket Oxford. Once I installed it as a default dictionary, I can't make default any one else for Spanish books.
What is kind of weird is that I have the English-Spanish Pocket Oxford, the Oxford Advance Learner, the Logman, plus the two Oxford dictionaries that comes with the Kindle and I can change among them with no problem for English books.
I was exploring the language metadata of the Spanish dictionaries with mobi2mobi, but they are correctly set to Spanish, and I don't know what exactly I have to modify to correct this behavior.