In September 1530, at Coburg, Martin Luther wrote his Sendbrief vom Dolmetschen, an open letter on translating. In it he explained where to find the right German word. Man muss die Mutter im Hause, die Kinder auf der Gasse, den gemeinen Mann auf dem Markt danach fragen und denselbigen auf das Maul sehen, wie sie reden. You must ask the mother at home, the children in the street and the common man at the market, and watch their mouths to see how they speak. The same letter gives an example. Luther did not translate the Latin Ex abundantia cordis os loquitur word for word. He wrote Wes das Herz voll ist, des gehet der Mund über, what fills the heart spills over from the mouth. Germans still use the line today.
Germans usually quote the principle as dem Volk aufs Maul schauen, to look the people on the mouth. That is a later short form. Luther’s own words were auf das Maul sehen.