The good news:
The Evil Genius created a nice Spanish-English dictionary for the Kindle ebook reader:
The bad news:
The dictionary is just what I was looking for to improve my Spanish. Unfortunately, it was created for Kindle 2, which uses only one dictionary at a time.
When using it on a Kindle 4 (which manages one dictionary for each language), it can be selected for English books only, which makes it pretty useless.
The actual problem is the missing language meta data of the dictionary.
After several unsuccessful attempts to change the language meta data, I discovered the Java Mobi Metadata Editor: https://github.com/gluggy/Java-Mobi-Metadata-Editor
It is easy to use and lets you change the "dictionary input" and "dictionary output" language meta data.
Thus, the dictionary now works on Kindle 4, Kindle Touch and Kindle Paperwhite.
If you're also interested in the updated version of the dictionary, you can download it.
It seems the dictionary cannot be used on the Kindle Fire: [link].