why this error apeared?

lkjt의 이미지

sorry, but i can't use hangul now...

locale: Cannot set LC_CTYPE to default locale: No such file or directory
locale: Cannot set LC_MESSAGES to default locale: No such file or directory
locale: Cannot set LC_ALL to default locale: No such file or directory

why this errors apperared?

Prentice의 이미지

apt-get install locales
dpkg-reconfigure locales