Custom localized JSON text

So far, JSON text (/tellraw etc.) allows showing localized texts from the game via the "translate" key, but there is no way to actually produce localized messages with custom content other than by using resource packs. I think something like this would be useful as an option:

"localize": {

  "en":"Welcome!",

  "de":"Wilkommen!"

}

In addition to "text", "translate" etc., this would select the correct content based on the user's language (both "en" and "en-US" should be supported, also the default value if no supported language is set). It also seems useful to be able to provide a nexted text object instead of a plain string, which would then work like as provided via "extra".