is there a filter for english only?

looking at http://search.twitter.com/operators it doesn't appear as if you can specify a language to limit your results to.

However, presuming your search terms are english words, its very likely that only english tweets will match.

If there are terms you want to exclude, you can start a word with dash (their example is "beer -root" which would give you beer related tweets that are not about root beer). This method obviously isn't perfect since if the word root is used elsewhere it would prevent you from getting a tweet you may be interested in.

Do you have an example of something you're trying to search for that doesn't work well? I'm sure we could probably come up with some suggestions.