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I have a Samsung Galaxy Grand Prime, which isn't exactly the most high-end Android/Samsung Smartphone out there, I do not have a lot of features that other people have, and I was simply wondering if there is any way to change the system language to Japanese. When I go into Settings > Language and input > Language, I only have the options shown in this image: 
Is there any way to change the language to Japanese or am I out of luck? Thank you in advance.
Background:
Firstly please take a look at the similar post: How do I add a new language?.
Adding a full language support for a device is done at ROM level, the langauges that the default keyboard will support are added before compiliation and the android keyboard will only give you the option to download dictionaries for the languages it supports..
Generally it is not easy to add a new language to the OS if it is not done by the manufacturer.
The best solution will be to flash a new ROM that holds your desired language, either a stock ROM made for your area by your device manufacturer or a custom ROM.
Having said that, nevertheless it could be that the ROM supports many languages but are hidden because of things like:
This behaviour has been noted for Sony, HTC and Samsung devices. Luckily there is a possible workaround in this case (yay!).
Since you have a Samsung device, you may try Language Enabler (Wanam) which is recommended on this XDA thread.
Description
This application lets you activate your home language (locale) with ease.
Language Enabler activate all supported Samsung languages and keyboard inputs.
Starting with Android 4.2, the permission to change your system language became protected.
If you are familiar with Android developments tools, then you can grant the required permission to the App without root, by executing this command through usb
adb -d shell pm grant com.wanam android.permission.CHANGE_CONFIGURATIONIf you have root access on your device then Language Enabler can automatically do it for you.
This application supports all languages / locales: Arabic, Chinese, Indian, Spanish, French, Hebrew, Farsi... عربية، عبرية، فارسية.
There are some devices variants (US,Chinese) that remove completely all unused languages, sadly there is no much thing we can do in this case.
Features :
Choose language to be enabled (83+ languages)
Display Languages on their Locale encoding
Important
This app doesn't add any language support, it just enable all supported languages, so if you don't get your local language after reboot, that means it's not supported in your ROM.
Please remember to share you feedback in comments section
adb devices and check if it is listed, After then perform the command adb -d shell pm grant com.wanamandroid.permission.CHANGE_CONFIGURATION
Jul 10, 2017 at 13:35
adb devices which it then came up with a gibberish identifier for my phone, which I am used to, and next to it it said device. So I ran the command adb -d shell pm grant com.wanamandroid.permission.CHANGE_CONFIGURATION again, just to find out there were two options for the Japanese language, and one was decidedly not Japanese, or it would not set. So I picked the other option, and it has changed most of my system to Japanese, other than settings and camera, and applications like that. So I would like to thank you for your help!
Jul 10, 2017 at 14:04
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: C:\Users\[USERNAME]\AppData\Local\Android\Sdk\platform-tools.
Or, just open Command Prompt (Terminal) and go to the root folder by typing the command below.
cd C:\Users\[REPLACE_WITH_YOUR_PC_NAME]\AppData\Local\Android\Sdk\platform-tools
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.
