Thank you for your comments, Nic.
Using a Kindle dictionary with an ebook hinges entirely on the ebook's language being correctly defined (as Spanish, in your case). Often, the tools people use to convert their own documents produce "English" foreign-language ebooks, which unfortunately prevents them from taking advantage of the dictionary lookup.
If you're willing to do a bit of non-user-friendly technical experimentation with the PDF->Kindle process, it might be possible get it to work. If you are interested, please contact us at "info AT intangiblepress.com" and we'll see if we can help you figure it out. Please describe how you're currently converting your PDFs in the email.