application_controller.rb

class ApplicationController
  before_action :set_locale

  private

  def set_locale
    if authenticated?
      I18n.locale = current_user.locale
    else
      langs = (request.headers['HTTP_ACCEPT_LANGUAGE'] || '').split(',').map { |l| l.split(';').first }
      I18n.locale = langs.find { |l| I18n.available_locales.include?(l.to_sym) }
    end
  rescue I18n::InvalidLocale
  end
end