It's no secret that the guy hated the standard German language and literature, and preferred to speak and write in French while in private.
Now, Frederick the Great imposing French on Prussia would be a step too far even for him, but what if he sponsored efforts to codify and standardize Low German - a process that had begun under the Hansa, only to fall apart - or even Old Prussian, since the language had not gone completely extinct by then yet? On a scale of linguistic curiosity to language revival, his efforts would probably fall more towards the former side but, who knows?
Now, Frederick the Great imposing French on Prussia would be a step too far even for him, but what if he sponsored efforts to codify and standardize Low German - a process that had begun under the Hansa, only to fall apart - or even Old Prussian, since the language had not gone completely extinct by then yet? On a scale of linguistic curiosity to language revival, his efforts would probably fall more towards the former side but, who knows?