Use esc_attr() to print month name in French

Here is my solution, which has been tested and works:

// Timestamp to format (defaults to current time)
$timestamp = time();

// Set language code
$dateformat = new IntlDateFormatter(
    'fr_FR', // Use valid Locale format
    IntlDateFormatter::LONG, IntlDateFormatter::NONE
);

// Set date format
$dateformat->setPattern('MMMM');

// Echo (and capitalize) formatted timestamp
echo ucfirst( $dateformat->format( $timestamp ) );

It seems that many valid locales can be found here: https://gist.github.com/jacobbubu/1836273