On last march Google and their AJAX Search and Feeds team launched the Language API. This new API boasts two functions, language translation and language detection. Maybe the the most important function of this API is the Translation. Is very easy to use it.
To see more information about this API and learn how to use it, please click here Google AJAX Search API Blog: Introducing the AJAX Language API - Tools for Translation and Language Detection
Or use this PHP script for language detection.