I don't think there's something you can configure. You would need to put something in your own URL Rewriter. What Deane suggests here for short URL is kind of similiar:
Seems like an unintended flaw. I'll see if we can do anything about it...
EPiServer Development Team
We have a site with multiple languages, English and Chinese. English is fallback for Chinese. The problem is that links are rendered with the language selection in the URL. When I view the site in Chinese all links to the fallback pages include /en/ in the URL. If I click a link to a fallback page the whole site changes language to English. I would prefer if only the content for that page was shown in English.
Is there a way to only output links in the current language even do pages do not exist in that language?