There are actually many places in Luther's Works where he refers to Speaking in Tongues. This is just one
"Also the fool doesn’t understand St. Paul’s words correctly when he writes of speaking with tongues (I Cor. 14[:2–29]). For St. Paul writes of the office of preaching in the congregation, to which it is to listen and to learn from it, when he says: Whoever comes forward, and wants to read, teach, or preach, and yet speaks with tongues, that is, speaks Latin instead of German, or some unknown language, he is to be silent and preach to himself alone.
97 For no one can hear it or understand it, and no one can get any benefit from it.
Or if he should speak with tongues, he ought, in addition, put what he says into German, or interpret it in one way or another, so that the congregation may understand him. Thus St. Paul is not as stubborn in forbidding speaking with tongues as this sin-spirit is, but says it is not to be forbidden when along with it
interpretation takes place.
Luther interprets I Cor. 14 to refer to speaking in foreign languages in the sense of Acts 2. Actually, I Cor. 14 refers to a form of ecstatic speech (
glōssalalia)."
Luther, Martin: Pelikan, Jaroslav Jan (Hrsg.) ; Oswald, Hilton C. (Hrsg.) ; Lehmann, Helmut T. (Hrsg.):
Luther's Works, Vol. 40 : Church and Ministry II. Philadelphia : Fortress Press, 1999, c1958 (Luther's Works 40), S. 40:iii-142