You selected the language for your phone during set up. To change it:
If you don’t see the language you want, tap 
Languages added to this list will also be available to use with the onscreen keyboard. You can also add languages to the keyboard that aren't system languages.
This device, when bought in India, supports text input in Hindi, English, Marathi.
To remove a language, go to Settings > System > Language & region > System Languages, then tap 

For specific apps, you can select a language that's different than your phone's default language:
Tap the app's name and select the language to use.
Not all apps support changing languages.
Set your preferences (temperature units, first day of the week) so that apps display information the way you want. Your default preferences are based on your selections for display language and app languages.
To change these preferences:
Adjust settings:
Not all apps support regional preferences.