Thanks for writing this great plugin. Unfortunately, I cannot use it since my site is multilingual.

has_shortcode( $post->post_content, 'review_slider') only checks in the post for shortcodes and does not detect shortcodes in widgets or do_shortcode('').

Hence, this is preventing the loading of the scripts in those cases:

function review_slider_enqueue_scripts(){

  global $post;
  if( !is_a( $post, 'WP_Post' ) || !has_shortcode( $post->post_content, 'review_slider') )
    return;

//enqueue styles and scripts ...
}
add_action( 'wp_enqueue_scripts', 'review_slider_enqueue_scripts');

Not sure how to exactly access the widget content cause I can’t see it in $wp_registered_widgets.