Per T183494#3855303
wgContentLanguage is deployment. Probably a bug in beta cluster config, although the widget should handle it more gracefully.
$wgContentLang on https://deployment.wikimedia.beta.wmflabs.org/ is deployment, causing various bugs. It probably should be en? (On the other hand, having a place to test whether everything handles an invalid content language gracefully is maybe not a bad thing?)