I agree with @mattbasta. Discussed this with a MDN contributor, we can just redirect to the locale-less version (english URL, but without the '/en-US' part), and then file a bug for MDN to handle proper internal redirections for the pages where they have translated the URL.

So, if I'm french, Going to /docs/patterns on Marketplace, I'll get a redirection to https://developer.mozilla.org/docs/Web/Apps/Design/Patrons_navigation_adaptative from Marketplace, and then MDN will redirect to https://developer.mozilla.org/fr/docs/Web/Apps/Design/Patrons_navigation_adaptative - possibly by having another redirect in the middle, but this would be handled fully my MDN