Sorry to butt in on your response to CV5, but I felt a need to answer this. I thought I explained this earlier, but I'll say it differently.
Paul did not say his mind was unfruitful for 20 years, that's conjecture. But he said if he speaks in an unknown tongue with no interpreter, his mind is unfruitful. This was a point he was making to rebuke the Corinthians about what they were doing. Yet, he said "I will pray with the Spirit, and pray with the understanding also." This implies that Paul's mind was indeed fruitful, because this statement was his practice. He wasn't saying that as some ideal he hadn't reached. He was saying it because it was his experience.
So, why did he not identify the language? Simply because he was making general statements about how the Corinthians needed correction. Therefore what language he spoke (or languages) was irrelevant to his argument.