Your Mac is set to display the system language in the region or country where it was purchased by default. However, you can change this if you want to. You can also change the language based on the apps that you use. Here is a guide that will tell you how to do both.
Changing the system language
Click on the Apple menu, which can be found in the top left of the menu bar.
From the pop-up that will appear on your screen, choose from the list of installed apps, and then select the language.
Click on ‘Add’.
Keep in mind that not every app will support the different languages. Once you have added an app to this list, you can change the language at any given time you want to. Also, by removing an app from the list, the app will again start to use the default language.
This is how you are able to change the languages used on macOS for both the entire system and only for specific and individual apps.