Please provide Scriptural support for a "prayer language".
Paul mentioned with his understanding and without. Works for me.
There are diversities of tongues. (1 Cor. 12:28) These passages refer to a private operation between an individual and God.
1 Corinthians 14:2 For he that speaketh in an unknown tongue speaketh not unto men, but unto God: for no man understandeth him; howbeit in the spirit he speaketh mysteries.
1 Corinthians 14:4 He that speaketh in an unknown tongue edifieth himself; but he that prophesieth edifieth the church.
1 Corinthians 14:13 Wherefore let him that speaketh in an unknown tongue pray that he may interpret.
1 Corinthians 14:14 For if I pray in an unknown tongue, my spirit prayeth, but my understanding is unfruitful.
1 Cor. 14 deals with the difference between this gift, the 'prayer language' as some call it, and the public gift of tongues and interpretation, which basically equals prophecy.
For the record, I don't believe any gift has passed away, and in fact won't be done away with until Jesus returns. In fact, we've seen the restoration of giftings occur as God restores the church to a state greater than the former house's glory, as the fullness of time approaches.