Different than the Manifestation of Languages
Prayer in other languages, the language given by the Holy Spirit at baptism into Him, is a free gift available to every believer who receives the Promise of the Father: the Holy Spirit.
Often this is confused with the spiritual manifestation of kinds of languages, one of the nine spiritual manifestations of the Holy Spirit (1 Corinthians 12:7-11). That manifestation of the Spirit is used through a believer as He wills. They cannot "operate it at will" because "all these work that one and the selfsame Spirit, dividing to every man severally as he will."
Consider this analogy:
A person's primary, native language (most likely English) can be spoken by a person any time they wish.
In the same way, the private prayer language given by the Spirit is operated the same way. It is a language a Spirit-filled believer can speak freely anytime they wish (1 Corinthians 14:5-18). The difference is, it is the Holy Spirit supplying the words of the language. They have not consciously learned it. They open their mouth, begin to speak, and He fills it with the words (Acts 2:4). It is a free gift they all receive when baptized in the Holy Spirit.
4They were all filled with the Holy Spirit, and began to speak with other languages, as the Spirit gave them the ability to speak. 39For the promise is to you, and to your children, and to all who are far off, even as many as the Lord our God will call to himself.” Acts 2:4,39
45They of the circumcision who believed were amazed, as many as came with Peter, because the gift of the Holy Spirit was also poured out on the Gentiles. 46For they heard them speaking in other languages and magnifying God. Then Peter answered, 47“Can anyone forbid these people from being baptized with water? They have received the Holy Spirit just like us.” Acts 10:45-47
6When Paul had laid his hands on them, the Holy Spirit came on them, and they spoke with other languages and prophesied. Acts 19:6
Now consider the manifestation of prophecy. It is the same language as their native language, yet they are not able to express it whenever they will. The Holy Spirit supplies the words, even though they are in their own language, whenever He wishes to use them in that manifestation (1 Corinthians 12:10).
This compares to the manifestation of kinds of languages. The Holy Spirit supplies the words, when He wishes to use them in this way. They cannot "do it at will" anymore than they can prophesy "at will" (1 Corinthians 12:30).
The manifestation of different languages is used to edify and build up the Assembly, when it is accompanied by the manifestation of interpretation of languages (1 Corinthians 14:5-6).
Therefore, it is intended to be used in the presence of other believers (1 Corinthians 14:13). For this reason, not everyone will necessarily be used by the Spirit in this way, and it is regulated in the Assembly for those who are (1 Corinthians 12:30-31).
18I thank my God, I speak with other languages more than you all. 19However in the assembly I would rather speak five words with my understanding, that I might instruct others also, than ten thousand words in another language. 26What is it then, brothers? When you come together, each one of you has a psalm, has a teaching, has a revelation, has another language, has an interpretation. Let all things be done to build each other up. 27If any man speaks in another language, let it be two, or at the most three, and in turn; and let one interpret. 28But if there is no interpreter, let him keep silent in the assembly, and let him speak to himself, and to God. 37If any man thinks himself to be a prophet, or spiritual, let him recognize the things which I write to you, that they are the commandment of the Lord. 38But if anyone is ignorant, let him be ignorant. 39Therefore, brothers, desire earnestly to prophesy, and don’t forbid speaking with other languages. 40Let all things be done decently and in order. 1 Corinthians 14:18,19,26,27,28,37,38,39,40
The free gift of the prayer language in other languages is used to edify and build up oneself (1 Corinthians 14:4) (Jude 1:20) (Isaiah 28:11-12).
It does not need to be interpreted, for it is directly edifying the one praying, because it is intercession by inspiration of the Holy Spirit.
26In the same way, the Spirit also helps our weaknesses, for we don’t know how to pray as we ought. But the Spirit himself makes intercession for us with groanings which can’t be uttered. Romans 8:26
2For he who speaks in another language speaks not to men, but to God; for no one understands; but in the Spirit he speaks mysteries. 1 Corinthians 14:2
For this reason, its use is confined to private prayer.
2For he who speaks in another language speaks not to men, but to God; for no one understands; but in the Spirit he speaks mysteries. 18I thank my God, I speak with other languages more than you all. 19However in the assembly I would rather speak five words with my understanding, that I might instruct others also, than ten thousand words in another language. 28But if there is no interpreter, let him keep silent in the assembly, and let him speak to himself, and to God. 1 Corinthians 14:2,18,19,28
Therefore it is not regulated by the Assembly (1 Corinthians 14:37-39).
when a believer understands the difference between these two supernatural uses of languages by the Holy Spirit, they will better be able to edify themselves and others, as well as discern when the order of God is being preserved.