I've never seen a rational or biblical justification to suggest that tongues are anything other than actual language.
The idea of a "private prayer language" is common in Charismatic and Pentecostal circles, but is foreign to Scripture.
I say this as someone who used to do the whole tongues thing back in my Pentecostal days (and for some years after); but I simply could not justify it with a meaningful and serious exposition and study of Scripture.
That said, because it was a learned behavior, it would be very easy for me to convincingly fake it (I'm not saying I used to fake it, I believed it was very genuine at the time).
-CryptoLutheran