Why am I not able to save / update data in wp_editor?

you’re using :

// only low case [a-z], no hyphens and underscores
$editor="contenten";

and then trying to get it by different name:

  if(isset($_POST['content_en']) && $_POST['content_en'] != '')

another thing is u using different keys like @vancoder said

also remember to sanitize the data :
https://codex.wordpress.org/Data_Validation

this code should work for u :

 function __construct() {
        add_action('edit_form_after_editor', array($this, 'custom_meta_box_lang'));
        add_action('save_post', array($this, 'save_en_content'));
    }

    function custom_meta_box_lang(){
        global $post;
        // set 'your_meta_key' to the actual key
        $content = get_post_meta($post->ID, 'content_en', true);

        // only low case [a-z], no hyphens and underscores
        $editor="contenten";

        // See my comment below
        $editor_settings = array(
                                'wpautop'       =>      true,
                                'media_buttons' =>      true,
                                'textarea_name' =>      'content_en',
                                'textarea_rows' =>      10,
                                'drag_drop_upload' =>   true);

        wp_editor( stripslashes( $content ), $editor, $editor_settings);

    }

    function save_en_content($post_id){
        if(isset($_POST['contenten']) && $_POST['contenten'] != '')
            update_post_meta($post_id, 'content_en', $_POST['contenten']);
        else
            delete_post_meta($post_id, 'content_en');
    }