Operator shelf edit form (bug 1074431)

r? @ngokevin @cvan @spasovski 
Like I mentioned in another comment somewhere else, because this is a preview, we want the language currently being edited, not the one the user is browsing with. Since we always start with en-US, this made sense, unless there's a way to use `|translate` to choose a specific language, which I've not seen before.