This has come to light in the logs because Thumbor doesn't care when the language in the lang parameter is the same as the wiki's, but Mediawiki treats this as an invalid request:
Nov 15 11:29:51 ms-fe1001 proxy-server: HTTP status code mismatch. Mediawiki: 400 Thumbor: 404 URL: http://upload.wikimedia.org/wikipedia/fr/thumb/1/1d/Logo_Naruto.svg/langfr-986px-Logo_Naruto.svg.png
Which makes me wonder if Mediawiki defaults to using the wiki's default language when no lang parameter is provided. Which Thumbor doesn't currently do, leaving the system's default locale (english).