For if I pray in tongues, my spirit is praying, but I don’t understand what I am saying. Well then, what shall I do? I will pray in the spirit, and I will also pray in words I understand. I will sing in the spirit, and I will also sing in words I understand.”
‭‭1 Corinthians‬ ‭14:14-15‬ ‭NLT‬‬

“But if no one is present who can interpret, they must be silent in your church meeting and speak in tongues to God privately.”
‭‭1 Corinthians‬ ‭14:28‬ ‭NLT‬‬

“So, my dear brothers and sisters, be eager to prophesy, and don’t forbid speaking in tongues.”
‭‭1 Corinthians‬ ‭14:39‬ ‭NLT‬‬

Praying in tongues is not the same thing as speaking in tongues, as defined in the scriptures above. When we do not know what to say when we pray, we pray in tongues. When we do this, this is not something that must be kept private as some churches teach. When we pray for others, it is often left up to God and the matter taken to Him from The Spirit. When we pray to God in Tongues, it's not necessary to interpret, just as the things we ask of God is something we ask to God Himself. We need to be able to get ahold of God when we pray for others. Churches that squash this, saying this is always a private matter are wrong.  

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.”

Some people say The Spirit groans and nothing comes from us. If you’ve ever preached a sermon that was God Inspired, there were times in the sermon when you said something that didn’t come from you, but you said it. This is because The Holy Spirit was speaking through you. The same goes for praying in The Spirit. The Holy Spirit, who resides in you, prays through you and it isn’t always actually words as some say it must be. The languages spoken by the Spirit aren’t always human languages, which God created anyway. There are other heavenly languages that we speak when praying or speaking in tongues. If God wants an unbeliever to hear you and understand you, then your prayer language, or prayer in tongues, will change according to God’s choice, not your own. This is what happened on the day of Pentecost. Many heard those, later called Christians, speak messages in their own language. This was for their sake. People tend to limit God’s power by putting their beliefs and restrictions on tongues. I’ve seen people oppressed by demons and nothing through normal prayer would do anything, and no demands in Jesus name was bringing any result. This was because we can’t deliver anyone, Only God can. When we started praying in tongues, then some strong holds were broken and the demons fled. I have personally witnessed this many times. When we pray in tongues it’s a one way communication to God, and when God answers, our demeanor gets powerful in speech, and sometimes even louder. I have seen things change when this starts to happen. Chains are broken!

I know of a large church that teaches that you pray in the Spirit at home, behind closed doors, privately.This is most likely because of this scripture:

Matthew 6:6

But when you pray, go away by yourself, shut the door behind you, and pray to your Father in private. Then your Father, who sees everything, will reward you.

See, right before this scripture, in Matthew 6 verse 5, it’s says:

When you pray, don’t be like the hypocrites who love to pray publicly on street corners and in the synagogues where everyone can see them. I tell you the truth, that is all the reward they will ever get.

The intent of the prayers matter!
Standing on a street corner, trying to look Godly, which means your heart is selfish, as the Pharisees did as stated in Matthew 5:5, is not the same as trying to help someone.
This is a trick of The devil to use scripture in a twisted way, as he did to Jesus in the wilderness. Jesus set him straight! When praying for others, we run out of things to say, or don’t know what to say. This is when we let The Holy Spirit, who resides in us, pray through us. God will answer.