Enforce locale settings by configuring defaults

Settings in /etc/default/locale take precedence over SSH client settings.
- On systems with undefined locales, explicitly define all LC_ settings.
- On systems using UTF-8 locales and no undefined locale problems,
  explicitly define LC_ settings required for consistency.
Install 1.9.3 ffi expected by older themes.
Merge pull request #448 from ar-jan/locales

Improve locales (follow-up)
Merge branch 'refs/heads/master' into 2.3.x-dev