Are there any good, open source engines out there for detecting what language a text is in, perhaps with a probability metric? One that I can run locally and doesn't query Google or Bing? I'd like to detect language for each page in about 15 million pages of OCR'ed text.
Not all documents will contain languages which use the Latin alphabet.
Depending on what you're doing, you might want to check out the python Natural Language Processing Toolkit (NLTK), which has some support for Bayesian Learning Algorithms.
In general, the letter and word frequencies would probably be the fastest evaluation, but the NLTK (or a bayesian learning algorithm in general) will probably be useful if you need to do anything beyond identification of the language. Bayesian methods will probably be useful also if you discover the first two methods have too high of an error rate.