What is praying in tongues? Is praying in tongues a prayer language between a believer and God?

Romans 8:26 teaches us, “In the same way, the Spirit helps us in our weakness. We do not know what we ought to pray for, but the Spirit himself intercedes for us with groans that words cannot express.” Two key points make it highly unlikely that Romans 8:26 is referring to tongues as a prayer language. First, Romans 8:26 states that it is the Spirit who “groans,” not believers. Second, Romans 8:26 states that the “groans” of the Spirit “cannot be expressed.” The very essence of speaking in tongues is uttering words.

That leaves us with 1 Corinthians 14:4-17 and verse 14 especially: “For if I pray in a tongue, my spirit prays, but my mind is unfruitful.” First Corinthians 14:14 distinctly mentions “praying in tongues.” What does this mean? First, studying the context is immensely valuable. First Corinthians chapter 14 is primarily a comparison/contrast of the gift of speaking in tongues and the gift of prophecy. Verses 2-5 make it clear that Paul views prophecy as a gift superior to tongues. At the same time, Paul exclaims the value of tongues and declares that he is glad that he speaks in tongues more than anyone (verse 18).

https://www.gotquestions.org/praying-in-tongues.html

Question: "Is speaking in tongues evidence for having the Holy Spirit?"

Answer: There are three occasions in the book of Acts where speaking in tongues accompanied the receiving of the Holy Spirit—Acts 2:4, 10:44-46, and 19:6. However, these three occasions are the only places in the Bible where speaking in tongues is an evidence of receiving the Holy Spirit. Throughout the book of Acts, thousands of people believe in Jesus and nothing is said about them speaking in tongues (Acts 2:41, 8:5-25, 16:31-34, 21:20). Nowhere in the New Testament is it taught that speaking in tongues is the only evidence a person has received the Holy Spirit. In fact, the New Testament teaches the opposite. We are told that every believer in Christ has the Holy Spirit (Romans 8:9; 1 Corinthians 12:13; Ephesians 1:13-14), but not every believer speaks in tongues (1 Corinthians 12:29-31).

So, why was speaking in tongues the evidence of the Holy Spirit in those three passages in Acts? Acts 2 records the apostles being baptized in the Holy Spirit and empowered by Him to proclaim the gospel. The apostles were enabled to speak in other languages (tongues) so they could share the truth with people in their own languages. Acts 10 records the apostle Peter being sent to share the gospel with non-Jewish people. Peter and the other early Christians, being Jews, would have a hard time accepting Gentiles (non-Jewish people) into the church. God enabled the Gentiles to speak in tongues to demonstrate that they had received the same Holy Spirit the apostles had received (Acts 10:47, 11:17).

https://www.gotquestions.org/tongues-Holy-Spirit.html