How to change LANG = en_US ?

[Foss User <fossist@gmail.com>]

I want that when Debian boots and I log into gnome and open
gnome-terminal, echo $LANG shows en_US instead of en_IN.

I found that in /etc/environment, the content was:

LANG="en_IN"

So, I changed it to LANG="en_US" and rebooted the system. But after
rebooting and opening gnome-terminal, I find echo $LANG to output
en_IN again.

So, where should I make this change so that after rebooting echo $LANG
outputs en_US ?