Words with umlauts can't be searched (å, ä, ö, etc.). This means that this software isn't useful to the majority of Europeans. If you only were to add umlauts, such as German, Swedish, and Danish umlauts (å, ä, ö, ü, ø, æ), then you would potentially have twice as many users.

M. Winther
http://home7.swipnet.se/~w-73784/index.htm

You could also make the html versions of the umlauts searchable. For instance, if the user searches an html document for a word that contains ä, then you could also return search results for the html umlaut code for ä.
/Mats