Does speaking in tongues mean speaking a foreign language or is it kind of a heavenly language that is unknown to men?
Not all speak a different language. some speak different sound.
It is speaking a foreign language only.
1 Corinthians 14:13-16
13 Therefore let one who speaks in a tongue pray that he may interpret. 14 For if I pray in a tongue, my spirit prays, but my mind is unfruitful. 15 What is the outcome then? I will pray with the spirit and I will pray with the mind also; I will sing with the spirit and I will sing with the mind also. 16 Otherwise if you bless in the spirit only, how will the one who fills the place of the ungifted say the “Amen” at your giving of thanks, since he does not know what you are saying?
1 Corinthians 14:27-28
27 If anyone speaks in a tongue, it should be by two or at the most three, and each in turn, and one must interpret; 28 but if there is no interpreter, he must keep silent in the church; and let him speak to himself and to God.
The ONLY place a “heavenly tongue” is spoken of is in 1 Corinthians 13 and it is used to illustrate the point that even if one COULD speak in the language of angels, that is not what Christians are to be about. They are to be about love. And love is described best in the whole of chapter 13 of the book of Corinthians. I recommend highly anyone reading it.
1 Corinthians 13:1 If I speak with the tongues of men and of angels, but do not have love, I have become a noisy gong or a clanging cymbal.