A localization mod is exactly what the UE4 modding support should be stellar at. Provided, of course, that your modding kit includes the po files and your modder understand UE4 well enough to import the translated version, set up a new language and cook it as a mod.
The basic problem with UE4 modding is just that - the cooking and patching infrastructure is quite powerful, but you need to understand every bit of it to truly offer mod support and spend weeks or months on it.