In addition to translating the book and textual portions of the website and standard library, as mentioned in the prior post, my proposal that the OP cited was to translate the more complex error message patterns produced by rustc. That suggestion was based on item 3 of two posts of mine on u.r-l.o.
As of last May, rustc had about 5000 error message templates. Translating even 1/3 of those would probably be a substantial help to new ESL (English as a Second Language) Rustaceans, particularly for errors related to traits and associated types, lifetimes, and the borrow checker.