Rubão, tá aí!

find_key_in_array.php

<?php

$where = array(
    "productcategory" => array(
        "id" => 410,
        "productcategorytranslation" => array(
            0 => array(
              "translation" => "Fahrräder (Profakt)",
              "id" => 952106
            )
        )
    );
    
$string = "productcategory.productcategorytranslation.translation";

$keyToFind = end(explode('.', $string));

function find($keyToFind, array $where) {
    foreach ($where as $k => $v) {
        if ($k == $keyToFind) {
            if (is_array($v)) {
                return current($v);   
            }
            return $v;
        }
      
        if (is_array($v)) {
            return find($keyToFind, $v);
        }
    }
}

find($keyToFind, $where);