Strings in class not being translated. (All other strings are.)

I’m still not sure why this problem occured. I’m guessing the class loads before the WordPress translation functions do.

I ended up circumventing the problem by using functions instead of variables:

    function ins_banister_right(): string {
    return __( "Rampe d'escalier à droite", 'co_product_configurator');
}
function ins_banister_left(): string {
    return __( "Rampe d'escalier à gauche", 'co_product_configurator');
}
function ins_none(): string {
    return __( "Aucun", 'co_product_configurator');
}

    $select_html .= '<div class="copc_banister_field"><label>'. $this->ins_banister_left() .'</label><select name="copc_banister_left">';
    $select_html .= '<option value="">'. $this->ins_none() .'</option>';

More details can be found in this post.