From a useuser experience perspective, it is never good to throw a user into a site completely in another language. If you change the language of the navigation on the user, they may not even be able to continue to use your site at all.
For the sanity of your users, I'd recommend showing the English content on the language site with navigation and template in the native language. I'd also recommend an apology notice translated into the native language:
We're sorry. This page hasn't been translated yet. We are working on it, but other section of the site are higher priority. For now, it is in English.
For search engines, this is not ideal. Google has requested that indexed pages be all one language. It recommends not mixing languages.
That means that you need to prevent Google from indexing the page. You could do so with one of two meta tags in the <head> section of your page:
- Your first option is to noindex the page:
<meta name="robots" content="noindex">. This will prevent Google from indexing the page at all. - Your second option is to use a rel canonical to point Google to the English version of the page:
<link rel="canonical" href="http://example.com/en/same-page.html">This will usually mean that when Google find the duplicate content it will choose to index the English version.
Of the two, the noindex is the stronger option. Google always obeys noindex, but in the last couple years Google has occasionally started to ignore canonicals.