Fix locale problems in Raspbian

If you get errros like this while running commands on your Raspbian:

perl: warning: Setting locale failed.
perl: warning: Please check that your locale settings:
        LANGUAGE = (unset),
        LC_ALL = (unset),
        LC_TIME = "es_ES.utf-8",
        LC_MONETARY = "es_ES.utf-8",
        LC_CTYPE = "es_ES.utf-8",
        LC_COLLATE = "es_ES.utf-8",
        LC_MESSAGES = "es_ES.utf-8",
        LC_NUMERIC = "es_ES.utf-8",
        LANG = "C"
    are supported and installed on your system.
perl: warning: Falling back to the standard locale ("C").

Or those:

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

You may have to edit /etc/default/locale and add the default locales:

LANG=en_GB.UTF-8
LC_ALL=en_GB.UTF-8
LANGUAGE=en_GB.UTF-8