Manual:$wgMetaNamespaceTalk

Details
The name of the meta discussion namespace. If set to false the value of $wgMetaNamespace (or $wgSitename if $wgMetaNamespace is set to false) postfixed with "_talk" is assumed. This is useful for different languages as spelling or grammar may differ.

probable bugs
Using this variable might cause critical problems when using mediawiki which have been installed in some languages like Persian. for more information you might want to see the bug report in bugzilla.