Magento 1.x
Mage::app()->getLocale()->getLocaleCode()
Magento 2
/** @return string */
function rm_locale() {
/** @var \Magento\Framework\ObjectManagerInterface $om */
$om = \Magento\Framework\App\ObjectManager::getInstance();
/** @var \Magento\Framework\Locale\Resolver $resolver */
$resolver = $om->get('Magento\Framework\Locale\Resolver');
return $resolver->getLocale();
}