[WordPress] How to define a new locale for sorting data.

using-setlocale.php

<?php

// Set a new locale, define string comparison functions to be affected,
// and store the original locale.
$original_locale = setlocale( LC_COLLATE, 'de_DE' );

// Do whatever you need to do for German sorting here

// Reset the locale to its original state
setlocale( LC_COLLATE, $original_locale );

using-the-collator.php

<?php

// Instantiate the Collator and pass it the locale
$col = new \Collator( 'de_DE' );

// Sort the array of last names.
$col->asort( $last_names )

// Done!