Greetings in the name of Jesus Christ the son of the living God.
Allow me share the word of God concerning the gift of the Spirit-Speaking in tongues.

Two Categories of Tongues

1 Corinthians 12:1, 8-10
(1) Now concerning spiritual gifts, brethren, I would not have you ignorant.
(8) For to one is given by the Spirit the word of wisdom; to another the word of knowledge by the same Spirit;
(9) To another faith by the same Spirit; to another the gifts of healing by the same Spirit;
(10) To another the working of miracles; to another prophecy; to another discerning of spirits; to another divers kinds of tongues; to another the interpretation of tongues:

Source

It is true that tongues are mentioned as one of the 9 spiritual gifts in 1Cor12:10. Spiritual gifts are distributed as the Spirit wills (1Cor 12:11). Spiritual gifts are manifested to bless or benefit the body of Christ as a whole (1 Cor 12:7). The divers kinds of tongues imply different types of supernatural or earthly unlearned tongues/languages that are spoken out loud in the public assembly. The gift of tongues has to be interpreted to give understanding to the hearers in the church.

However, we are going to discover that there is a difference between tongues as a public ministry gift and tongues as a private prayer language. We fall into error when we confound these two categories of tongues.

The two categories of tongues are:

(i) The public ministry of tongues or the gift of divers kinds of tongues
(ii) The private prayer language of tongues

Tongues as evidence of Holy Ghost baptism

We are going to examine the instances in the scripture where believers received the Holy Ghost baptism and what kind of evidence followed. Jesus compared the Holy Spirit to wind (John 3:8). You can not see the wind however you can see the effects or evidence of the wind when the tree branches are seen to shake.

1. 120 Jewish believers on the day of Pentecost

Acts 2:4
(4) And they were all filled with the Holy Ghost, and began to speak with other tongues, as the Spirit gave them utterance.
The 120 believers who received the baptism of the Holy Spirit on the day of Pentecost all spoke in tongues as evidence of this infilling.

2. Gentiles gathered at Caesarea in the house of Cornelius

Acts 10:44-46
(44) While Peter yet spake these words, the Holy Ghost fell on all them which heard the word.
(45) And they of the circumcision which believed were astonished, as many as came with Peter, because that on the Gentiles also was poured out the gift of the Holy Ghost.
(46) For they heard them speak with tongues, and magnify God. Then answered Peter,
Peter received a divine commission to come and preach to Cornelius, a Gentile and Roman army captain, plus his friends and family gathered at his house in a city called Caesarea. In the midst of the sermon the Holy Ghost fell on these Gentiles and they began to speak in tongues. Peter and the 6 Jews who had accompanied him clearly heard these Gentlies speaking with tongues and they acknowledged it was a sign of them being filled with the Holy Ghost (Acts 11:15).

3. Twelve believers at Ephesus

Acts 19:2, 6
(2) He said unto them, Have ye received the Holy Ghost since ye believed? And they said unto him, We have not so much as heard whether there be any Holy Ghost.
(6) And when Paul had laid his hands upon them, the Holy Ghost came on them; and they spake with tongues, and prophesied.
Apostle Paul asked the believers at Ephesus whether they had received the Holy Ghost baptism ever since they had got saved. They replied that they had not received Him. Apostle Paul went ahead to lay hands on these 12 Gentile Christians and the Holy Ghost came on them. The evidence of the Holy Ghost baptism upon their lives was that they spoke with tongues.

From these proof scriptures above we can safely conclude that the initial physical evidence of receiving the Holy Ghost baptism is speaking in tongues.

Image source

Tongues as a private prayer language

1 Corinthians 14:4
(4) He that speaketh in an unknown tongue edifieth himself; but he that prophesieth edifieth the church.
As I mentioned earlier there is a difference between tongues as a public ministry gift and tongues as a private prayer language which is also an evidence of Holy Ghost baptism. The one who has been baptized in the Holy Ghost and speaks in tongues privately will edify or build up himself. This is in contrast to the public ministry of the gift of tongues where one builds up the church at large.

1 Corinthians 14:2
(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.
The private nature of the second category of tongues is emphasized in this scripture by explaining that the person praying in tongues is speaking to God and not man. This again contrasts with the first category of tongues which is a public ministry speaking to men after being interpreted.

1 Corinthians 14:5
(5) I would that ye all spake with tongues…
Whereas not every believer will operate in the public ministry of tongues, Apostle Paul expects every believer to speak in tongues as a private prayer language given to them upon receipt of the Holy Ghost baptism. The Holy Ghost baptism is available to every Christian for the asking. God is not a respecter of persons but He is a respecter of faith.

Stay a blessing. Thanks
@wanasoloben