"Tongues is a language understood by someone somewhere but the one speaking does not understand because it will be a language he hasn't learned - For one who speaks in a tongue does not speak to men but to God; for no one understands, but in his spirit he speaks mysteries. 1 Cor. 14:2 NASB - the point this is making is that when someone speaks in tongues, he does not understand what he is saying . . . which goes along with 1 Cor. 14:14 - For if I pray in a tongue, my spirit prays, but my mind in unfruitful."
Nowhere in Paul’s letter does it imply that the speaker does not understand what he himself is saying; it’s those listening to him who do not understand, as they do not speak his language. For this reason, he does not ‘speak to men, but to God’ – to those listening to him, even though he is praying from deep within his heart (i.e. ‘in the spirit’) he is speaking ‘mysteries’. ‘Mysteries’ here is just an idiomatic expression like the modern expression “it’s all Greek to me/us”.
“1 Corinthians 14:14 is probably the main text used to argue that the language speaker did not understand his language. Paul says that if he should speak in a language (without translation), "my spirit prays but my mind is unfruitful [akarpos]." Lenski takes akarpos as passive: "my nous or understanding" is inactive and thus akarpos--"barren," "unfruitful," producing no distinct thoughts".
Paul could however have been using akarpos in the active sense:
A decision upon its meaning centers in akarpos ("unfruitful") whether the adjective is passive in sense, meaning the speaker himself receives no benefit, or active in sense, meaning his nous (understanding) provides no benefit to others...The view that assigns akarpos a meaning of "produces nothing, contributes nothing to the process"... is not convincing, because akarpos does not mean "inactive." It is a word for results and does not apply to the process through which the results are obtained. The present discussion does not center on the activity or nonactivity of the tongues speaker's mind, but rather on potential benefit derived by listeners.
The whole context of 1 Corinthians 14 is the effect upon the hearers of untranslated languages.
Paul’s concern is the edification of the group. Therefore, 14:14 should be taken as "My spirit prays but my mind does not produce fruit [in others]."
Or, the fact that I understand what I’m saying is unfruitful to others, in that they can’t benefit from it.
“If I pray in an unknown language (i.e. unknown to those listening to me), my spirit prays, but my understanding (the fact I know/understand what I’m saying) is unfruitful (to those listening to me).”
The “my sprit prays” part can be taken two ways: “my spirit prays (but theirs doesn’t)” or “my spirit prays (i.e. though I’m praying from deep within my being)”, my understanding (the fact that I understand what I’m praying/saying) is unfruitful (does not benefit those that are hearing me pray/speak).
Paul, being (for his time anyway) a ‘world traveler’, would have probably spoken several languages; maybe not all fluently, but certainly enough to ‘get by’. So, yes, he certainly did speak ‘in tongues’
If I may paraphrase from what another poster wrote, and kind of hit the nail on the head (with apologies for hijacking his reply) .....
You have been taught the concepts and their definitions to support a preconceived conclusion of what tongues are 'supposed to be' and not strict adherence to the reality of life in a multi cultural, multi-lingual community such as Corinth and the common everyday issues encountered with respect to communication and mutual intelligibility. I am not trying to be mean or unkind, but there's absolutely nothing in these passages that imply anything but real language.