Add translations

If you want to add another language:

1. create a file in /config/locales/[2-char-language_shortcode].yml
2. fill in all defined locale variables in the new language (see existing language files for keys)
3. run bundle exec rspec spec/i18n_spec.rb to verify missing keys and interpolation consistency
4. configure the new translation in config/config.yml (default_locale)
5. fork the repository and make a pull request