Heroku does not seem to be loading config/locales/pt.yml. (Language is being set correctly to pt.)
config/locales/pt.yml
pt
I18n is working perfectly on localhost,
Writing this line will also fix the problem in your application.rb
config.i18n.load_path += Dir[Rails.root.join('my', 'locales', '*.{rb,yml}').to_s]