Grace and truth are the foundations and orders for speaking the language of God. Grace is God's ability, and truth is the force that ignites God's ability. Grace and truth came from Jesus Christ (John 1:17 KJV). He is the language of truth because He is the word of truth.
Truth can either be spoken in our understanding or without our understanding in other tongues, as the Spirit of God gives them utterance. "And they were all filled with the Holy Spirit and began to speak with other tongues, as the Spirit was giving them utterance" (NASB 1995). No other language is accepted or received in the kingdom of God except truth.
Psalm 33:4 says, "The Word of the Lord is right, and all God's works are done in truth."