SilverStripe Controller that sets the default locale on all controllers upon init. Helps resolve translation issues.

ControllerExtension.php

class ControllerExtension extends Extension
{
	public function onBeforeInit()
	{
		// Ensure the locale is set properly on all requests (otherwise AJAX requests might return templates rendered
		// with wrong locales).
		i18n::set_default_locale(Config::inst()->get('i18n', 'default_locale'));
	}
}

Apply in your config file:

Controller:
  extensions:
    - ControllerExtension