Change default locale in Debian Linux

GNOME

In GNOME - System > Administration > Language Support

KDE

In KDE - System Settings > Regional & Accessibility > Country/Region & Language


Command line

Commands:
dpkg-reconfigure locales
locale-gen en_US.UTF-8

Files

Default locale: /etc/default/locale
/etc/locale.gen
/usr/share/i18n/SUPPORTED
/var/lib/locales/supported.d/local