My client has text in their URL which are the same name as the Mention Me parameters. This means that despite them passing me locale=en_GB in the tag we ignore this and override it with the locale=uk-uk that we see in their requesting page URL.
You can select the second option in the advanced options area of the edit brand view. This will allow you to only accept parameters from a requesting URL if it has the format "mm_" first e.g. "mm_locale=."