Nowdays there's no need for using separate i18n gem, on plain Rails 3.0.6 and above (5.0 included) installation, fallbacks
value can be one of the following:
# application.rb
# rails will fallback to config.i18n.default_locale translation
config.i18n.fallbacks = true
# rails will fallback to en, no matter what is set as config.i18n.default_locale
config.i18n.fallbacks = [:en]
# fallbacks value can also be a hash - a map of fallbacks if you will
# missing translations of es and fr languages will fallback to english
# missing translations in german will fallback to french ('de' => 'fr')
config.i18n.fallbacks = {'es' => 'en', 'fr' => 'en', 'de' => 'fr'}
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