So, everything works fine locally, but at heroku it changes to :en
Fire up console on both environments:
Heroku:
heroku console --app myapp
What version of Ruby on Rails are you using? I'm not positive about 2.x, but in Rails 3 you can set the default locale in config/application.rb using config.i18n.default_locale = :en. (This is the line, commented out by default.)
config/application.rb
config.i18n.default_locale = :en