Actor Pierre Niney was not a German speaker, so his co-star Paula Beer taped his lines in German and he learned them by ear, according to director François Ozon.
In German, the titular name is usually spelled "Franz". The film's title, "Frantz", is a frequent misspelling by French speakers, so the title serves as a nod to the fact that the titular character is supposed to be a francophile and therefore he might have preferred that spelling, not to mention it sounds similar to "France".