I'm aware that this is an old post, but I've decided to leave my answer here. So if anyone in the future is stuck, they can reference from my post.
Most of articles that I've found bait you to install and buy their apps.
I don't trust those apps and I sure don't want to pay any money for it.
So I've put together what I've found based on 2 reliable sources, including the answer up there.
(Contents are pasted down below)
Source: How to add a new language that isn't provided by manufacturer on Android
1. Enabling the developer mode:
Developer mode allows you to gain access to more controls, but it's hidden by default.
So in order to enable Dev mode, do the followings:
Open your Device (Phone/Tablet) > Settings > Device/Phone Information > Tap on Build Number 7 times.
2. Enable USB Debugging:
In order for your PC to control your device, you need to enable something called USB Debugging and that can be found in Dev Mode
Go back to the Settings and Search up "Developer Options".
In the Dev Options, scroll down until you see "USB Debugging" option and enable it.
3. Using adb commands on PC:
Now you have to connect your Android Device to your PC using an USB cable.
Then download Android Studio from Google.
You can verify your install location:
Or, just open Command Prompt (Terminal) and go to the root folder by typing the command below.
Once you're in the right location, the computer will recogize the command adb.
You can start using adb commands. Before we get into any further, we have to go back to our Android Device.
4. Download Language Installer App:
There is an app called Language Enabler by Wanam.
After installing the app, run the following commands on your command prompt.
This command will give the app root access to your device to make changes.
adb -d shell pm grant com.wanam android.permission.CHANGE_CONFIGURATION
5. Langauge Installation:
Now all you have to do is to install the language using the Language Enabler app.
Go back to the app, select the language you want to install and press install.
This'll install the language and you can now delete the app and undo everything up there as we no longer need them.