A quick google search provided:
You could always roll your own as well. Grab a dictionary file, and lookup each word in the tweet to compare against the dictionary. If a significant number of those words exist in the dictionary, there’s a high chance it’s English. Of course, probability comes into play, and you’ll need to take into account certain variables such as completely ignoring hashtags, as a non-English user may use English hashtags to convey their message. You’ll have to determine a confidence ratio you’re comfortable with. (For fun, you may also want to pick up non-English dictionaries and look for words that don’t match the English dictionary to see what language it actually is from, and then build a list of known words that are not English to help filter faster.)
Depending on how many tweets you’re looking to check, option 1 is most likely the simplest.