The Evil Genius created a nice Spanish-English dictionary for the Kindle ebook reader:
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 senseless.
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 my Kindle 4 and the Kindle Touch. If you're also interested in the updated version of the dictionary, you can download it from: