I believe something that is overlooked is this...People think that there is only 1 gift of speaking in tongues (meaning when talking on the subject of tongues, most people refer to 1 Cor 11 (chapter on 9 spiritual gifts or abilities given by the Holy Spirit). But there are actually 2 different types of praying or speaking in tongues. 1 Cor chapter 11 is dealing with a gift of speaking in tongues and a gift of interpretation of tongues, and these gifts are given ONLY as the Spirit wills. Not everyone has these gifts. I believe every believer CAN by faith choose to step into a realm and relationship with God where we are all able to be used in these ways.
But now take a look at Mark 16:15-18 - Jesus says, "...And these signs shall follow them that believe; ...they shall speak with new tongues..."
So Jesus is saying that there will be certain signs (not gifts) that will accompany the believer. I believe that if you are a believer, speaking in tongues is something that God wants to bless you with. This is different than the GIFT of speaking in divers tongues. Also, note Romans 8:26 and Jude 20-21. In Jude, it says to build yourselves up on your most holy faith, praying in the HOly Spirit. We know that 'praying in the Spirit' is refferred to as praying in tongues, because Paul said in Corinthians, "I pray with the spirit, and I will pray with the understanding also:" - 1 Cor 14:15.
So now back to my main point. God would not only give SOME believers an ability to edify themselves in the HOly Spirit. He gives that gift to all who will believe and receive it. But the gift of speaking in divers tongues (1 Cor 11) is only given to some. So this 'speaking in tongues' is a prayer language that is willing to give to every believer in order to communicate with God and to edify and build themselves up. I believe that you can pray in tongues when you are with other believers. Obviously we should be led by the integrity of the Word of God and the leading of the Spirit, but let's remember, that where the Spirit of the Lord is there is liberty.
So to answer the thread, you can pray in tongues (and you should) in private. Just like you received the gift of speaking...you can open your mouth and talk or pray in english anytime you want, right??? It's the same with praying in tongues, you can openyour moutha nd pray in toungues anytime you want (obviously following the Spirit's leading if in public).
Praise God!!! Love you with all tender of affections of Father!