With Day One 2 and later, we've added localization support for many different languages. If you have enabled multiple foreign languages and keyboards on iOS, you can manage what language is used in Day One by doing the following:
Go to your device Settings > scroll down to find Day One > Preferred Language.
This setting affects Day One only, not your entire device language setting. That can be done in the device Settings > General > Language & Region or different keyboards can be added in Settings > General > Keyboards.
Go to System Preferences > Language & Region.