Replace special characters with “normal” ASCII characters during indexing

<?php

// Add the following to your theme's functions.php

add_filter( 'facetwp_index_row', function( $params, $class ) {
    if ( 'aufsichtsbehoerden' == $params['facet_name'] ) {
        $raw_value = $params['facet_value'];
        $params['facet_value'] = iconv( 'UTF-8', 'ISO-8859-1//TRANSLIT//IGNORE', $raw_value );
    }
    return $params;
}, 10, 2 );