Between August and November 1532 Luther delivered a series of lectures on psalm 45, part of a longer series on selected psalms. Rorer took notes, using shorthand and these were later published, though Luther said he spoke to his students as the words came to him, and so the published record was not as polished as he would have liked.
The prelegomena to psalm 45 are in volume 12 of the St Louis edition of Luther's Works, to which I cannot find a link. The relevant part, on page 198-9, is qouted by Westerholm as follows:
Even if this language [Hebrew] were useless otherwise, one should still learn it out of thankfulness. It is part of religion and divine worship to teach and learn this language through which alone we can learn anything of the divine. In it we hear God speak, we hear how the saints call upon God and achieve the mightiest deeds; thus study directed toward learning this language might rightly be called a kind of mass or divine service.
The Latin can be seen in the Weimar edition, volume 40 part 2, page 474, from line 18.
Although Luther believed strongly the Bible should be available in the German of the ordinary people, he did not think that German could fully convey the original meanings, or was adequate for ministers.