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 shows praying with understanding to be prayer in native language. it's a contrast from native language to tongues respectively when it says with understanding and in the spirit.
You folks are mistaking him to say it is a contrast between being in the Spirit OF GOD or in the flesh. In the spirit here is not IN THE SPIRIT OF GOD. It does not mean not in the flesh. It simply means not with understanding. MY HUMAN SPIRIT prays when I pray in tongues. It is a lower case "S" in English to indicate human spirit.
Paul said MY SPIRIT prayeth.
1Co 14:14 KJV For if I pray in an unknown tongue,
my spirit prayeth, but my understanding is unfruitful.