I am French and use my Debian system in French.
However, I would like to have error messages in terminal in English. It allows me to post them on English speaking websites.
How can I change my locale to do so? I precise I don't want to have all my system in English.
You have to change LANG and LANGUAGE variable in current shell.
Put
export LANG=en_US.UTF-8 export LANGUAGE=en in your shell rc file ( for bash it is ~/.bashrc ) and restart the terminal session ( or just source the rc file one more time with source ~/.bashrc or even just restart bash with exec bash )
Note, that you have to generate this locale before.
Also you can use export LANG=C.
Also there are other LANG variables (from man bash):
LANG Used to determine the locale category for any category not specifically selected with a variable starting with LC_. LANGUAGE Used to set messages languages (as LC_MESSAGES) to a multi-valued value, e.g., setting it to fr:de:en will use French messages where they exist; if not, it will use German messages, and will fall back to English if neither German nor French messages are available. LC_ALL This variable overrides the value of LANG and any other LC_ variable specifying a locale category. LC_COLLATE This variable determines the collation order used when sorting the results of pathname expansion, and determines the behavior of range expressions, equivalence classes, and collating sequences within pathname expansion and pattern matching. LC_CTYPE This variable determines the interpretation of characters and the behavior of character classes within pathname expansion and pattern matching. LC_MESSAGES This variable determines the locale used to translate double- quoted strings preceded by a $. LC_NUMERIC This variable determines the locale category used for number formatting. 
LANG environment variable just for setting (preferred) language of programs run in console is wrong. That's what LC_MESSAGES and LANGUAGE environment variables are meant for. See The 'LANGUAGE' locale variable - how to set English as primary language? A bug in gettext?. LC_ALL=C should be used instead with unset LC_ALL to reset. See unix.stackexchange.com/q/87745/168003 You can call an application with the following syntax:
For example you want the English manpages (for just a single command):
user@host:~# LC_ALL=C man ls Or you run into an error (my system is in German):
user@host:~$ ls -la /root/ ls: Öffnen von Verzeichnis /root/ nicht möglich: Keine Berechtigung user@host:~$ LC_ALL=C ls -l /root/ ls: cannot open directory /root/: Permission denied 
LC_ALL=C instead of LC_ALL=c which fails too for me with message bash: warning: setlocale: LC_ALL: cannot change locale (c) If you just want the error messages and not the whole locale, you can use:
export LC_MESSAGES=C
Try referencing this: 1) https://wiki.debian.org/ChangeLanguage
In short, you want to set your LOCALE (via LANG), preferrably in your .bashrc.
export LANG=en_US.UTF-8 
export LANGUAGE=English