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 usbadb -d shell pm grant com.wanam
android.permission.CHANGE_CONFIGURATION
If 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، Ùارسية.
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
Q & A