I have found a couple of issues regarding the easy_words.txt for the Spanish language:

• It contains single letters (e.g. m, c, b ...) that are not recognized as words in Spanish.
• It contains acronyms, whereas the easy_words.txt list for English does not.
• It contains words that could be considered difficult.

I think this could be solved by standardizing the way words are added to this list by some criteria, so everybody knows which type of words this list is going to contain.