<?php
global $all_text_domains;
add_filter( 'plugin_locale', function( $locale, $domain ) {
	global $all_text_domains;
	$all_text_domains[] = $domain;
	return $locale;
}, 10, 2 );

add_action('admin_menu', function(){
	add_options_page('TextDomains', 'TextDomains', 'administrator', 'textdomains', function(){
		global $all_text_domains;
		echo '<pre>';
		print_r( $all_text_domains );
		echo '</pre>';
	});
});