How do I get the right entry data based on locale and slug?
Probably the CRAFT_LOCALE isn't set correctly. There is an in-depth knowledge article here.

To solve this, you could add a conditional to the /public/index.php:

// Path to your craft/ folder
$craftPath = '../craft';

//Locale
if ( preg_match("/\/nl\//i", $_SERVER['REQUEST_URI']) ) { //Check if /nl/ is in the uri
    define('CRAFT_LOCALE', 'nl_nl');
}
else {
    define('CRAFT_LOCALE', 'en_us');
}