Fix redirection to default language (en)

The redirection from `/index.html` to the default language subdirectory (/en) for an unknown language does not work.

An unknown language leads to a nonexistent object property, which returns `undefined`, but the test checks for equality to `null`. (Maybe the behavior for missing properties has changed in the past???) The proposed change should work both in the case that the unknown property results in `undefined` or `null`.

Note that I did not (yet) include `public/index.html` in the commit. Also, the `gh-pages` branch is not updated.
What's the workflow for that?