Would love to see how one of the interfaces to Google Translate would do on our user guide.
For example here is a python program to translate text of an epub using Google Translate, and that allows custom translate word dictionaries that a user could create that would include proper names like Sigil, BookBrowser, Preview, CodeView, url, etc.
https://github.com/quantrancse/epub-translator
Might make a good plugin for Sigil that would at least jump start the translation process and hopefully make it easily editable in Sigil to get things close.
That said all of the user guide images would have to be redone to have the correct user interface language.