This is useful when you need to install apps that require precise locale settings..
From a terminal run:
nano /etc/locale.gen
You will get something like this
en_GB ISO-8859-1
en_GB.UTF-8 UTF-8
en_GB.ISO-8859-15 ISO-8859-15
en_US.UTF-8 UTF-8
# aa_DJ ISO-8859-1
# aa_DJ.UTF-8 UTF-8
# aa_ER UTF-8
# aa_ER@saaho UTF-8
# aa_ET UTF-8
Valid locale settings are en_GB.UTF-8 , en_US.UTF-8