Important: You do not need to change the system language to work with different languages in Office 2016 for Mac. For example, you can:
Office 2016 for Mac can be used with multiple languages, all of which are installed by default. The Office for Mac user interface (menu items, buttons, or other interface elements) and Help content change dynamically based on your operating system preferences, so one way to change the language in Office for Mac is to change the language of your operating system. For example, if you set the operating system language preference to French, Office for Mac will be displayed in French.
If you still want to change the language of your operating system, follow these steps:
Click the Apple menu, and then click System Preferences.
Click Language & Region.
Click the + sign under Preferred languages, select a language from the list, and click Add.
When asked if you want to make this language your primary language, click Use [Language Name].
You can also set a primary language by dragging a language to the top of the Preferred languages list.
You may have to restart your computer for language changes to take effect.