I hit the same problem.
The suggested workaround sounded a bit long winded so I deleted
/etc/mediawiki/LocalSettings.php then went to
http://my-server/mediawiki/mw-config to generate a new one.
Installing the new LocalSettings.php was enough for it to spring back
into life.
I then restored my old LocalSettings.php and verified it was back to
broken state.
Next I added the line
$wgLanguageCode = "en-gb";
to LocalSettings.php and it was back up and running.
Finally I switched back to the new LocalSettings.php because my old one
was so old (2012 I think) that I figured it would cause more pain in the
future.
