From IRC:

<mat> so that works for english, but what about other languages :/
<mat> I was about to say you should redirect to 'https://developer.mozilla.org/%s' % uri, but I noted that most (all?) of these pages don't exist in some languages, e.g. french
<mat> even though we have the translation : https://marketplace.firefox.com/developers/docs/patterns?lang=fr vs https://developer.mozilla.org/fr/Apps/Design/Responsive_Navigation_Patterns

IHMO, we need to a) redirect to the URI without the lang b) make sure MDN has all the content translated in all languages we used to support for these pages before switching over.