do not send an overriden language if none is set

If the browser does not have any language configured
`window.navigator.languages == []` and
`window.navigator.language == ""`.
(at least on FF)
In this case an empty GET parameter is submitted to `/api/l10n/index?locale=`.
This leads to a 400 Bad Request response and no language data.
Therefore the navbar has no UI elements and is unusable, make login impossible.