Your App might have this issue if you use multithreading server. It's not a bug Rails, but on your app if you don't put
before_filter to all controllers. Open this Pull Request for the details https://github.com/rails/rails/pull/34356, https://github.com/rails/rails/issues/34043, and https://github.com/svenfuchs/i18n/issues/381.
To prevent this issue, change how the way you set
I18n.locale = locale code to be in
around_filter block, not
around_filter :switch_locale def switch_locale(&action) locale = params[:locale] || I18n.default_locale I18n.with_locale(locale, &action) end
before_filter have became
before_action in Rails 4