Strings: plural to singular

Question

I'm working on an English to Spanish translation program and am stumped on a method.
The following method is supposed to take a plural Spanish noun and change it to a singular Spanish noun. In my limited understanding of Spanish, a word will end in "es" or "s". First I test if it ends in "es", and if it does I chop off the last two letters. This works for things like calcetines -> calcetin. However, there are words that end in 'e' without being plural, cause things like clases -> clas instead of clase. To fix that, I made it check through all the Spanish words in my array. If the word, chopped, with an 'e' matches one of the words, I add an 'e' on the end. This part doesn't work, as far as I can tell. The rest of the code is just for if it end with a plain old 's'.
     String SPluralToSingular(String word) {
          if (word.substring(word.length()-2, word.length()).equals("es")) {
               word=word.substring(0,word.length()-2);     
               for (int i=0;i<spanishWord.length;i++) { //Check if it wrongly killed 'e'
                    if ((word+"e").equals(spanishWord)) word+="e";
          } else if (word.charAt(word.length()-1)=='s') {
               word=word.substring(0,word.length()-1);
          return word;
Sample outputs:
mesas -> mesa (correct)
calcetines -> calcetin (correct)
clases -> clas (BAD!)

Answer 1

This is a list of nouns, so things like adios do not apply.
I know that I can't chop off the 'es' and do nothing else. It doesn't work when the 'es' was the result of a noun ending with an 'e' being put into plural form. If I can detect when a word exists that would make it true that the above went wrong (word without 'es' + 'e' is a valid word), then I can add the 'e' back on.