本文摘自 https://wiki.debian.org/ChangeLanguage ，感谢作者
* First, you have to set environment variables such as LANG, LANGUAGE, LC_CTYPE, LC_MESSAGES to your local language. Usually LANG (or LC_ALL) is sufficient. (shamelessly stolen from Claws Mail doc 
Check what you have at present with:
# env | grep LANG
* Second, you may also need to reconfigure your locales.
To do this, as root:
First: issue on a root terminal #export LANG=(the two letter code for your country).UTF-8 That is, for the Spanish language it would be:
# export LANG=en_US.UTF-8
Second: reconfigure locales issuing the following command:
# dpkg-reconfigure locales
A window will ask you to select the languages (you select with SPACE) you want to have available. Choose your own.
Changes may not show immediately, a reboot will be needed.
(Please feel free to modify these instructions if you know better. Thanks.)