The whole interface is translated, and the language is yours to pick regardless of what your device speaks.
Switching
Open Settings, go to the General tab and pick from the Language list: English, Español, Français, Português, Deutsch, Italiano, Türkçe or 日本語. The app switches immediately, no restart needed.
What changes and what does not
Menus, buttons, settings and messages all switch. Your notes are your notes, they stay exactly as written.
The AI side
The chat assistant answers in whatever language you write to it, independent of the interface language. If you want it to always answer in a particular language, say so once in custom instructions.
Dates too
Dates and times across the app follow your date and time format settings, so a German interface with an international date format, or the reverse, is perfectly possible.