The last line reads the locale settings and sets $LANG, which overrides the language settings and prevents users from changing their language. In order to make this work, /etc/pam.d/sshd will have to be changed, which will let users use a non-system default language.
Part of the problem is /etc/pam.d/sshd:
auth required pam_env.so # [1] /etc/default/ locale
auth required pam_env.so envfile=
The last line reads the locale settings and sets $LANG, which overrides the language settings and prevents users from changing their language. In order to make this work, /etc/pam.d/sshd will have to be changed, which will let users use a non-system default language.