This application allows you to activate all languages ​​supported by Samsung

It works without any intervention on your part until Jelly bean (4.2.2)

From Jelly bean (4.2.2) this app will need CHANGE_CONFIGURATION permission to be able to change the system language

If you know the adb command, you can download the Adb tools on your PC and grant the CHANGE_CONFIGURATION permission manually to the application using, through your computer by connecting the smartphone via a cable, the following command:

adb shell pm grant com.jkdevandroid.languefrancais android.permission.CHANGE_CONFIGURATION

Info: Check on google for more details

If your smartphone has root access, the app can get permission automatically for you

As of Android 7 Nougat, the app will ask for a new permission to be able to write the system configuration.

Some smartphone manufacturers completely remove unused languages ​​from the system, in which case there is nothing we can do.

We will be delighted with your suggestion :)