Hey. I admittedly can't give you too much great advice, but found your question interesting. I own a Kindle 2 and am always interested in the limits of the Kindle. I just did a quick check on Spanish language Kindle books, and they list almost 2,000 books (though I haven't done an intense search to see what kind or quality of books they are and I don't know their availability for non-US owners). If you are interested in seeing the selection, go to Amazon's main book site (not Kindle Books) and do an advanced search. Choose the format as Kindle and the language to Spanish, and then you can judge whether these books would in any way appeal to your mother.
It seems like it's still a little early for foreign language books in the e-pub world, but as demand grows, so too will the selection. Good luck though!