When adding a title that is in a foreign language, follow these basic rules regardless of citation style:
For German, capitalize the first word and all nouns.
For French, capitalize THROUGH the first noun in the title.
For Italian and other languages, capitalize just the first word.
(NOTE: Always capitalize all proper nouns.)
When citing a non-English reference work, use the original title followed by the English translation of the title in brackets.
Kluge, Friedrich. (2002). Etymologisches Wörterbuch der deutschen Sprache [Etymological dictionary of the German language] (24th ed.). Berlin, Germany: de Gruyter.
These resources will attempt to format your citation for you. They will get you probably 80% of the way there, but mistakes happen! Double check the citation with a handbook.