isReliable A boolean representing whether or not the detection interval believes the language code is reliable for the given text.
confidence A numeric value between 0-1.0 that represents the confidence level in the language code for the given text.
I wish there were more information on the methods being used to determine reliability and confidence. Details aren't necessary but I tried out their test page there and keyed in the Spanish word for Hello, Hola. Response:
SPANISH ( not reliable : 0.09097684)
Hmmm. Initial response on Google AJAX Language API: NOT RELIABLE Or at least not building much confidence yet.