The answer to this question is a very simple and practical one.
The key to understanding this is found earlier in the same chapter, 1 Cor 14:5, where Paul imagines a group of people, some of whom speak in various languages. For illustration, suppose a speaker speaks Mongolian but find themselves among a group of English speakers. If such a person gives a testimony in Mongolian (as a result of a miraculous gift of God) then no one will understand. Thus, in V5 we have:
I wish that all of you could speak in tongues, but I would rather have
you prophesy. He who prophesies is greater than one who speaks in
tongues, unless he interprets so that the church may be edified.
In V10, 11 Paul makes the same point in another way:
Assuredly, there are many different languages in the world, yet none
of them is without meaning. If, then, I do not know the meaning of
someone’s language, I am a foreigner to the speaker, and he is a
foreigner to me.
Lastly, allow me to quote the last four verses of this chapter (with comments by myself) as Paul concludes:
22 Tongues, then, are a sign, not for believers, but for unbelievers [so that new fields can be entered and evangelised].
Prophecy, however, is for believers [in their regular services to
share God's Word], not for unbelievers.
23 So if the whole church comes together and everyone speaks in tongues, > and some who are uninstructed or some unbelievers come in,
will they not say that you are out of your minds? 24 But if an
unbeliever or uninstructed person comes in while everyone is
prophesying, he will be convicted and called to account by all, 25
and the secrets of his heart will be made known. So he will fall
facedown and worship God, proclaiming, “God is truly among you!”
Lastly, to the trickiest part of the question: "he pray that he may interpret". Let us begin with a concrete example where the language is not at issue. Examine the example of Dan 8:27 which is the conclusion of the the vision of the Ram and Goat. Daniel records what he saw and Gabriel's explanation and concludes:
I was appalled by the vision; it was beyond understanding.
Thus, Daniel could not understand what had been revealed to him. I believe it is possible to have a revelation from God, even when the recipient understands the language, that the meaning is still obscure.
To continue the above hypothetical example, if one person receives a divine revelation, which is in a language he/she understands, namely Mongolian but is unknown to the rest of the group, that does not mean that even when translated, the revelation could be necessarily understood. Thus, Paul urges people to pray that they might have understanding.