This is defined in site.ini ( ContentObjectLocale ). You need to set up a siteaccess for the translation. This is the default translation. If you need another translation you need to use the translation button in eZ publish.
So you would need to set up a siteaccess for every language to support this.
E.g.
ez.no/norwegian
ez.no/english ez.no/german
Then if you e.g. would post a forum message for any of these languages it would be stored in the correct locale.
