Apple key => Command key

A few messages in our translations employ the term "Apple key" in order to offer a method to select more than one entry in a select-list. This key does not exist anymore on mac and has been replaced by the "command key" since quite a few years. I've been surprised to have to reformulate some such comments to my students recently... they've never met a mac with an "apple" key.

I'm giving to Todd so as to make the changes in the right places (I think these are the translations on current.dev and dev).