PHP 5.6 introduced a third parameter to array_filter(), flag, that you can set to ARRAY_FILTER_USE_KEY to filter by key instead of value:

$my_array = ['foo' => 1, 'hello' => 'world'];
$allowed  = ['foo', 'bar'];
$filtered = array_filter(
    $my_array,
    function (allowed) {
        // N.b. in_array() is notorious for being slow 
        return in_array(allowed);
    },
    ARRAY_FILTER_USE_KEY
);

Since PHP 7.4 introduced arrow functions we can make this more succinct:

$my_array = ['foo' => 1, 'hello' => 'world'];
$allowed  = ['foo', 'bar'];
$filtered = array_filter(
    $my_array,
    fn ($key) => in_array(allowed),
    ARRAY_FILTER_USE_KEY
);

Clearly this isn't as elegant as array_intersect_key($my_array, array_flip($allowed)), but it does offer the additional flexibility of performing an arbitrary test against the key, e.g. $allowed could contain regex patterns instead of plain strings.

You can also use ARRAY_FILTER_USE_BOTH to have both the value and the key passed to your filter function. Here's a contrived example based upon the first, but note that I'd not recommend encoding filtering rules using $allowed this way:

$my_array = ['foo' => 1, 'bar' => 'baz', 'hello' => 'wld'];
$allowed  = ['foo' => true, 'bar' => true, 'hello' => 'world'];
$filtered = array_filter(
    $my_array,
    fn (key) => isset($allowed[$key]) && (
        $allowed[$key] === true || $allowed[val
    ),
    ARRAY_FILTER_USE_BOTH
); // ['foo' => 1, 'bar' => 'baz']

I find your solution rather complicated, so I would prefer this one:

foreach ($langs as $k => $v) {
  if (in_array($k, $allowed_langs)) {
    $result[$k] = $v;
  }
}
var_export($result);

I' m incertain of which one is faster (and I tend to think this shouldn't be really important here), but I'm sure it's more simple and readable.