When you use Plone with either LinguaPlone (on its latest versions) or plone.app.multilingual in a multilingual site, Plone creates one folder for each language, so you will always have /en/foo/bar like URLs for your english content and /es/foo/bar for your spanish content, the URL /foo/bar will not exist.
You can, if you want, create "Redirect Aliases" so if users go to /foo/bar can be redirected to /en/foo/bar. If this is what you want you have it by default on Plone 5.2 (rc2 as of today) going to Actions -> URL Management submenu of the toolbar, and in previous versions of Plone you can install Products.RedirectionTool add-on.
As an alternative to @erral simple approach you can write an custom error page for the site root, catching 404s and instead redirect to /en/PRIORURL (and there are other implementation paths to get the same as well).
There you just need to override the method called attempt_redirect.
But make sure, to call the inherited method first and if there is a redirect, use this redirection and only otherwise follow with your custom logic. Ignoring this, you will loose Plone's ability to follow redirects (caused by renames or movement of content).