Since GS 3.0 you can use $LANG = 'xx_XX'; in gsconfig to set the default language (used in the login page)
http://get-simple.info/forums/showthread.php?tid=1877
http://get-simple.info/wiki/plugins:i18n#front-end
http://get-simple.info/forums/showthread.php?tid=1877
http://get-simple.info/wiki/plugins:i18n#front-end