This is Richard Stoneman’s paraphrase of a German source:

Frederick the Great . . . has strong views as to how these improvements to the German language shall be effected. For a start, something has to be done to prevent the further corruption of German taste by the appalling plays (die abscheulichen Stücke) of Shakespeare, which have already been translated into German. Not only do these plays not observe the unities, but they allow the mixing of classes on stage: kings and gravediggers may appear in conversation! Frederick’s second recommendation is the imposition of a national or core curriculum on all professors and philosophers: ‘In my view, one should prescribe to every professor precisely the rules which he is to follow in his lectures.’ He proceeds to do so; the rules include the detail that the professor must denigrate the philosophy of Epicurus, defend Galileo, and say nothing at all about Locke.

(Richard Stoneman, “‘A Crazy Enterprise’: German Translators of Sophocles, from Opitz to Boeckh”, Chapter 13 (pages 307-29) of Sophocles Revisited, Essays presented to Sir Hugh Lloyd-Jones, edited by Jasper Griffin, Oxford, 1999, at 310. Stoneman’s footnote on the royal quotation refers to page 81 of H. Steinmetz (ed.), Friedrich II, König von Preussen, und die deutsche Literatur des 18. Jahrhunderts: Texte und Dokumente, Stuttgart, 1985.)