Twig: show the name of the months based on the current locale

I'm just trying to show the name of the months based on the current locale.

{{ event.date|date('F') }}

but the months are always shown in english...

I have tried this code below I found here, but the result is the same...

class Helper_Twig extends Twig_Extension
{
    public function getFilters()
    {
        return array(
            'datetime' => new Twig_Filter_Method($this, 'datetime')
        );
    }

    public function datetime($d, $format = "%B %e")
    {
        if ($d instanceof \DateTime) {
            $d = $d->getTimestamp();
        }

        return strftime($format, $d);
    }

    public function getName()
    {
        return 'Helper';
    }
}

NOTE: In the controller I'm checking the current locale using $request->getLocale and it corresponds to the locale parameter I'm switching in parameters.yml.

What is the problem?