<?php

$customEmails = array(
    'DE' => 'deutsch@email.com',
    'BG' => 'bulgarian@email.com',
    'RU' => 'russian@email.com',
);

$email = "default@email.com"

$country = $_POST['country'];

if (in_array($country, array_keys($customEmails))) {
    $email = $customEmails[$country];
}

/* ---------------------- */

mail($email, ..............);