I have not tried doing this, but it looks like there is a localization folder in the engine that you could potentially add your own.
For example:
C:\Program Files (x86)\Epic Games\UE_4.15\Engine\Content\Localization
I have not tried doing this, but it looks like there is a localization folder in the engine that you could potentially add your own.
For example:
C:\Program Files (x86)\Epic Games\UE_4.15\Engine\Content\Localization