Q. How does one cite a book with a bilingual title—e.g., a book where the full title is presented in both German and English? Thank you very much.
A. Use either an equals sign or a slash between the two forms of the title (with a space before and after the equals sign or slash); otherwise, such a source would be cited like any other work of its type. But if the source itself does not use a slash or other mark between the two titles, an equals sign should be preferred:
Appelbaum, Stanley, ed. and trans. Five Great German Short Stories = Fünf deutsche Meistererzählungen. A Dual-Language Book. New York: Dover, 1993.
Note that sentence-style capitalization is used for the German title, according to which German adjectives (like deutsche) are lowercase (see CMOS 11.6). Also note the series title in the example (“A Dual-Language Book”), which is optional (see CMOS 14.123).