How one theme can have two textdomains and still work with translation systems.

one-theme-two-textdomains.php

<?php
/**
 * The purpose of this file is to show how a theme can have multiple textdomains 
 * and still work with a single translation file.  Translation tools like the
 * one used on WordPress.org and Poedit are going to simply scan the project for
 * text strings, regardless of the textdomain (and for good reasons that are 
 * not relevant here).
 *
 * The code below works with that system.  It assumes that we have a single MO 
 * file based on the theme's textdomain.  So, how can two textdomains work? It 
 * does this by assigning the loaded strings in the MO file to the framework 
 * textdomain (remember that the translation system captures all strings no matter 
 * the textdomain).  That way, when a string is called using the framework 
 * textdomain, the string will be properly translated.
 */

add_action( 'after_setup_theme', 'jt_load_textdomains' );

function jt_load_textdomains() {

	// Load theme textdomain.
	load_theme_textdomain( 'jt-theme', get_template_directory() . '/languages' );

	// Load the framework textdomain.
	load_textdomain( 'jt-framework', '' );
}

add_filter( 'override_load_textdomain', 'jt_override_load_textdomain', 10, 2 );

function jt_override_load_textdomain( $override, $domain ) {

	// Check if the domain is our framework domain.
	if ( 'jt-framework' === $domain ) {
		global $l10n;

		// If the theme's textdomain is loaded, assign the theme's translations
		// to the framework's textdomain.
		if ( isset( $l10n[ 'jt-theme' ] ) )
			$l10n[ $domain ] = $l10n[ 'jt-theme' ];

		// Always override.  We only want the theme to handle translations.
		$override = true;
	}

	return $override;
}