use baberlei/assert library to internationalise your validation messages

index.php

<?php

/** @see https://github.com/beberlei/assert */

require __DIR__ . '/vendor/autoload.php';

use Assert\Assertion as A;

$sLang = 'en';

$aReturnMsg = [
	'en' => [
		A::INVALID_KEY_ISSET => 'could not find key!',
		A::VALUE_EMPTY => 'value not filled in!',
		A::INVALID_EMAIL => 'email is not valid!'
	]
];

try {
	A::keyIsset($_REQUEST, 'email');
	A::notEmpty($_REQUEST['email']);
	A::email($_REQUEST['email']);

	header('Content-type: text/plain');
	echo "hi, " . explode('@', $_REQUEST['email'])[0];
	die;

} catch (\Assert\AssertionFailedException $e) {
	echo $aReturnMsg[$sLang][$e->getCode()] . "\n";
}