IIRC there was a recent change that the browser is no longer launched with
a http(s): URL that points to the Jupyter server, but instead with a file:
URL that points to a local HTML file containing a redirect to the http(s):
URL. Maybe this change somehow interacts or interferes with browser
The language used in the UI depends on the settings in your browser.
Firefox's language settings look like the attached picture. It will use the
highest language in the list for which it has translations. At present I
don't think there's any way to override this in Jupyter to use a language
