If you change the default language of your iPad or iPhone to English, French, German, Italian, Japanese, Russian, Simplified Chinese or Spanish using the Settings app then Notes Writer user interface (menus, options, messages, etc.) will change automatically to that language as well the next time you open the app.
You can also change the keyboard language but this will not change the user interface language. You can set several languages on your keyboard to type in different languages simultaneously.
If you are using iOS 13, there is a new setting to change the default language of each app. Open Settings app then tap the "Notes Writer" icon and set the Language.
For more details about how to change languages in iOS, please, read these instructions: