What the Language referred to in 1Corinthians 14 is
There are questions about whether or not the word "unknown" is actually in 1Corinthians 14, in the original Greek or not.
This is not a major issue, but minor. One, just because of the content itself, it does not change Salvation through faith in Christ Jesus alone, or any other main foundational doctrine, but that of the gifts/graces/charis-ma grace-results, of Holy Spirit.
Also, the word unknown not being in the Greek, doesn't change whether we know it was a known/human language or unknown heavenly language, because Paul says explicitly that the tongue/supernatural language is not known by anyone naturally, "for no one understands him, but he utters mysteries in the Spirit," and so "let him pray that he may interpret his supernatural language," or else "let him speak between himself and God," and if an unbeliever or unlearned person walks in and a whole assembly is speaking in supernatural languages "will they not think you are out of your mind," so "let one speak at a time and two or three interpret, that all may be edified." This is in contrast to the manifestation of this grace/gift of Holy Spirit in Acts 2 where they all spoke in known human languages that were unknown to them, still a supernatural language, but different manifestation.
Also, we see different functions of heavenly/Holy Spirit supernatural languages such as "edification" 1Cor. 14, "deep groanings...Holy Spirit helps us when we know not what to pray," Romans 8, etc.
Most English translations such as ESV, NIV, NKJV, etc. all depict the main essential meanings of salvation (justification, healing, deliverance, protection) etc. and other foundational doctrines, they can taken to anyone who speaks English and they can understand how to be saved, the most important thing overall. But when it comes to specific nuances and word meanings, yes there is difficulty, because English is huge and Greek isn't and Greek is a picture language/one word means multiple things, where in English we have different words for the one Greek word, hence many English translations. The main point, though, is, is Holy Spirit on the words we read and speak? Is the Spirit of Life making those words life, as in Jesus' words that were "spirit and life," John 6. How much Presence, of God, do we have? Are we walking all the fullness of all the fruit (9 listed in Gal. 5) and all the gifts (9 listed in 1Cor. 12) of Holy Spirit, His whole Person? That is the center, not perfectly accurate pictures and word meanings. I pray and release that your, and mine, and all our eyes be opened to see God and His kingdom and truth and grace and all these things, in the Name of Jesus Christ, amen.