Use the facetwp_i18n hook for translations without WPML or the Multilingual add-on

<?php

add_filter( 'facetwp_i18n', function( $string ) {

    // Get the current site's locale
    $lang = get_locale();
    
    $translations = [
        'fr_FR' => [
            'All categories' => 'Toutes les catégories',
        ],
    ];
 
    if ( isset( $translations[ $lang ][ $string ] ) ) {
        return $translations[ $lang ][ $string ];
    }

    return $string;

});