[Doitsu]

Originally Posted by RayArdia

Having had some problems downloading and installing [...] Mobi Metadata Editor in the Firefox Plugins sub-directory.

Mobi Metadata Editor is a stand-alone Java program. There's no need to install it in the Firefox Plugins directory.
You should be able to run it by entering the following in a terminal window (after using cd to go to the folder that the .jar file is in):

java -jar MobiMetaEditorV0.16.jar

Originally Posted by RayArdia

BTW I tried a Spanish Kindle book from www.mobileread but that produced the same result as before, ie English dictionary used.

In that case there's something wrong with your Kindle or the Spanish dictionary, because the MR book that I linked to has the correct language metadata and worked on my Kindle. (If you used a different Spanish MR book, it might also have the wrong metadata.)

Do you want to use the default monolingual Spanish Kindle dictionary or did you install a custom dictionary?
If you've installed a custom dictionary, it might not have the correct language codes or is broken.

Try the following:

a) Delete the default monolingual Spanish dictionary and re-download it from Amazon.
b) Delete any custom dictionaries and re-install them on the Kindle by copying them via USB.
c) Double-check with Mobi Metadata Editor that all custom dictionaries have the correct Input/Output dictionary languages and all Spanish books have the correct language codes.
d) Restart the Kindle.
e) Select the monolingual Spanish dictionary as the default dictionary and try to look up a word in the MR book that I linked to.

If all of this didn't help you might have to reset the Kindle to the factory default settings. (Make sure to make a backup copy of all books on the Kindle first, because everything on the Kindle will be deleted.)

Note that you'll also need to re-register your Kindle after a reset.