Add a filter to a method in the parent theme

Try with init hook

add_action('init', 'child_theme_hooks');

function child_theme_hooks() {
    add_filter(array('Raymond_NewsletterWidget', 'get_widget_form'), 'translate_subscribe_btn');
}

function translate_subscribe_btn() {
    $new_form = str_replace('Subscribe', 'إشترك الآن', $form);
    return $new_form;
}