Fallback to "en" when the localized doc is not found

Try to fallback to "en" when the requested documentation is not
found for the selected language, and switch the language.
This ensures that some documentation is always found, like
KHelpCenter does.

This does not match the request of the bug (selection of
the available languages for each document) but it should help
until the site is restructured.
CCBUG: 359822