It is great that one can enter translations for user-defined blocks but it isn't clear how to add translations of comments. And in particular the top-level comment that becomes the 'help' for the block really needs to be localized. Is there a way to enter translations of comments?
No, not so far. Someday...
I think I'm on the right track to finding out how to change the text of the comment with the JavaScript function reporter; this: 
works, but if I put it in the say block, I get TypeError: this.block.topBlock is not a function.
Load the getter/setter library and use this script: 
Yeah, you figured it out before I had a chance to test it, but when I saw the resize icon inside the speech balloon I figured "contents" was something other than plain text. :~D