Changing simple things, like the names of items/blocks as well as their respective descriptions, could easily be done through modding.
I only know some basic french, so I would not be of much help, but modpacks that serve translation purposes could very well be done in a day and then uploaded to a file sharing website for others to use.
As for menu items and the like, I'd imagine one would have to modify that through C#, and I'm not sure on the difficulty of that, but it could very well be a possibility?