1 Corinthians 14:14-15 For if I pray in an unknown tongue, my spirit prayeth, but my understanding is unfruitful. (15) What is it then? I will pray with the spirit, and I will pray with the understanding also: I will sing with the spirit, and I will sing with the understanding also.
The context above shows praying in tongues to be something in which our understanding is unfruitful. IOW, praying WITH understanding is not praying in tongues. We cannot even understand what we are praying, ourselves, when we pray in tongues.
Notice he said HIS SPIRIT is praying when he prayed in tongues. That is what must be carried over into the next verse when we read about [praying in the spirit. He said that was contrasted from
his understanding. So, if he prayed in tongues and his spirit prayed and his understanding was unfruitful, then the next verse speaking about praying in the SPIRIT is praying in tongues! And praying with the understanding is contrasted from praying in tongues, and refers to praying with understanding which means praying in our native language. We can understand our own native language. We see that by contrasting praying with understanding from praying in spirit. So, praying in the spirit cannot refer to praying in our native language. Context disallows that in these two verses.