Yes, speaking in tongues is evidence of salvation.
But, that is not one of its' purposes.
The initial purpose of tongues is to give evidence of (one who is already saved)
receiving the baptism with the Holy Spirit by Jesus Himself
usually for some special ministry.
One needs to look at the original meaning of the word tongues:
The Hebrew noun lashon means "tongue", and as in many languages, "speech" or "language".
Where was tongues/languages first used? Genesis 11
Genesis 11:9 Therefore is the name of it called Babel; because the LORD did there confound the language of all the earth: and from thence did the LORD scatter them abroad upon the face of all the earth.
Acts 2:4..is speaking of Pentecost where his disciples received the Holy Ghost and was able to speak multiple languages in order to preach His message to the masses.
Religion and the Churches have expanded upon this as a way to be saved. Nothing could be further from the Truth.