Some CSL citation styles, such as the Chicago Manual of Style styles, convert titles to Title Case. However, Title Casing is specific to English. To prevent non-English titles from being Title Cased, specify the language of the corresponding item in your Zotero library using the “language” field.
Use two-letter language codes, e.g. “de” for German, “fr” for French, or “ja” for Japanese (four-letter codes can also be used, e.g. “de-DE” and “ja-JP”; see https://github.com/citation-style-language/locales/wiki for a list of locale codes). English items can be marked as such using “en”, “en-GB” (British English) or “en-US” (American English).
Titles should generally always be stored in Sentence case; Zotero can automatically transform titles into Title Case, but items cannot be reliably transformed to Sentence case (e.g., while treating abbreviations and proper nouns correctly).