Nice work!

I would like to contribute my Simplified Chinese translation to U, can I?

Line 80 of display.php should be:
<a style="float: right;" href="<?php echo wp_login_url( get_permalink() ); ?>" title="Login"><?php _e('Login', 'djd-site-post') ?></a>

and

Line195 of display.php should be:
<label for="djd_quiz" id="djd_quiz_label"><?php echo $no1; ?> <?php _e('plus', 'djd-site-post'); ?> <?php echo $no2; ?> =</label>

and

Line 135 of the main file:

public function register_plugin_scripts() {
		wp_enqueue_script( 'djd-site-post-script', plugins_url( 'djd-site-post/js/display.js' ) );
		wp_enqueue_script( 'djd-site-post-ajax-script', plugins_url( 'djd-site-post/js/script.js' ) );
		//adding 2 lines for better i18n
		$data = array( 'update_string' => __( 'Update','djd-site-post' ) );
		wp_localize_script( 'djd-site-post-ajax-script', 'djd_site_post_i18n', $data );
	} // end register_plugin_scripts

and

Line 8 of the script.js:

/*	var theLanguage = jQuery('html').attr('lang');*/
/*	if (theLanguage == 'de-DE') {
		var btnMsg = 'Aktualisieren';
	} else{
        var btnMsg = 'Update';
    }*/
var btnMsg = djd_site_post_i18n.update_string;

for i18n.

adding the following style to display.css may be better:

#djdsitepostcontent_ifr{
border: 1px solid #9f9f9f;
}