What language to use for error-pages on other-language site

A website I maintain has two possible languages, namely Dutch (the default), or English (if the url starts with <site>/en/<page>). We've only recently implemented full-site internationalisation, so now we've run into a problem that our 404 error is still in Dutch.

This caused the web developers to have a discussion what the best strategy would be. We've come to the following options:

• Detect the language of the user based on the URL, and display the error in that language
• Display the Dutch message, with a link at the top to "scroll down for English version", or display the messages side-to-side
• Display the detected language, and include a link to view the message in the other language

The problem with the first option is that this doesn't catch people going to "<site>/thispagedoesntexist", even though it is always possible for people to insert random gibberish in urls. In those cases, we don't display the correct language for English users.

The problem with the second option is that this may not look as clean as displaying a single language.

The problem with the third option is that it requires the user to waste another click on something that isn't going to lead them anywhere. It also doesn't quite feel right to me to intentionally link to a url that doesn't exist.

So what would be the best way to handle this? Perhaps somebody has a 4th option that evades all of these issues?