As per https://www.mediawiki.org/wiki/Manual:Using_custom_namespaces
the string values in $wgExtraNamespaces have to use underscores
instead of spaces ... otherwise MediaWiki wrongly recognizes
"Proposal_talk:Foo" to be in the main namespace.
$wgNamespacesWithSubpages[NS_PROPOSAL] = TRUE;
$wgContentNamespaces[] = NS_PROPOSAL;
define('NS_PROPOSAL_TALK', 3001);
-$wgExtraNamespaces[NS_PROPOSAL_TALK] = 'Proposal talk';
+$wgExtraNamespaces[NS_PROPOSAL_TALK] = 'Proposal_talk';
$wgNamespacesWithSubpages[NS_PROPOSAL_TALK] = TRUE;
$wgNamespacesToBeSearchedDefault[NS_LANG_DE] = TRUE;