All Good Frenchmen speak GermanH-France recently had a discussion about Fustel del Coulanges' "five languages of France" (here and here (seven messages in total)). At the time he wrote, German was written in Alsace, but dialect was spoken. Furthermore, it is not clear that he wanted to include German/Germanic language as a national language.
I have commented in the past that literature written in German by Alsatian writers ought to be considered French literature. The issue should not be limited to questions about language and national identity. As Lucien Febvre noted, Allemagne did not necessarily connote Germany (as it does in contemporary French), but eastern France: some part of the Holy Roman Empire, east of the Soane and west of the Rhine, that belonged to the kingdom of France and that spoke German. The argument was used for centuries to expand eastward and to allow the monarchy to stick its nose into imperial affairs.