I believe the easiest way is also the simplest, just like in linux: simple *.txt files containing the entire translation of the interface. Revit interface is so vast, parameteres names are so many, i don't think a single person could handle it ALL. Automated translation services (like Google translate) are highly unreliable.
I believe translation should be collaborative by means of online translation management like Weblate, Transifex, Damned Lies for Gnome, etc. There is only one way to translate it: open-source translation by many users around the world. In the linux world this is called localization, and it works great for many famous linuxes.
P.S. I had a very similar problem while using Allplan (a long time ago): the base version interface was in german (NOT english), and many items in the library weren't translated to english. It was the most stupid thing i've ever seen: 50% of the interface was localised (in all european languages), another 30% was in english, and the rest (20%) was exclusively german. Many commands had impossibly long names (translated from german of-course).
Many words should be translated context-wise ! Words without context could make no sense.
The translation file should be like a keynote file: one single *.txt file for each language. For reasonable management manner. E.g. the "width" word -> translate it once, then find&replace it in the entire file (entire interface).