I updated the opening post to give more details on the idea. It would only be a partial language conversion. All the buttons would remain in English, but hitting F1 while pointing the mouse at one could reveal its function in any language. Changing the font and size could be important for kanji use, so I'd include that.
So far, I haven't heard a convincing argument for making the editor convertible. If the language barrier is an issue for potential designers, then it's also barring them from this forum. I should probably hunt down people like Alt-F4 who've made levels for non-English speakers and ask their opinions. ... But, that also sounds like a big time sink, and I've only a couple months left to develop KS mods.