Making wp-content/languages/themes/ translation files accessible to Codestyling Localization via symlinks. Symlinks are made on the fly, should be update-proof. In our case, this is in a site-specific Must-Use plugin.

gistfile1.php

/* TODO 
	Using the 'current_screen' hook means symlinks will be recreated *every time* a Codestyling Localization page is called.
	A better solution would be to store themes' versions in a site_option and run this only when installed versions are higher than what we have in our site_option (i.e. an update has been made). See here:
	http://make.wordpress.org/core/2010/10/27/plugin-activation-hooks-no-longer-fire-for-updates/
*/

add_action( 'current_screen' , 'uneq_cs_translations' );

function uneq_cs_translations( $current_screen ) {
	/* Re/Create symlinks when visiting Codestyling page */
	if ( 'tools_page_codestyling-localization/codestyling-localization' == $current_screen->id ) {
		uneq_translation_symlinks();
	}
}

function uneq_translation_symlinks() {
	# TODO : user WP_filesystem instead of PHP filesystem functions
	$installed_themes = wp_get_themes();
	# NOTE : for completeness, should actually loop into all installed locales, not just the one currently used. We don't care because our install is fr_FR only.
	$locale = get_locale();
	foreach ( $installed_themes as $theme_id => $theme_object ) {
		# Theme Foundry themes all load_theme_textdomain from a languages/ directory
		if ( 'The Theme Foundry' == $theme_object->get('Author') ) {
			# Paths to the real translation files, in wp-content/languages/themes
			$mo_file_path = trailingslashit( WP_LANG_DIR ) . 'themes/' . $theme_id . '-' . $locale . '.mo';
			$po_file_path = trailingslashit( WP_LANG_DIR ) . 'themes/' . $theme_id . '-' . $locale . '.po';
			# Path to theme translations directory (wp-content/themes/theme/languages) 
			$theme_lang_dir_path = trailingslashit(WP_CONTENT_DIR) . 'themes/' . trailingslashit($theme_id) . 'languages/';
			# Create 'languages' directory if it's not already there
			if (! file_exists($theme_lang_dir_path)) {
				mkdir($theme_lang_dir_path,0755);
			}
			# Creating the symbolic links
			if (file_exists($mo_file_path)) {
				$mo_link = $theme_lang_dir_path . $locale . '.mo';
				symlink( $mo_file_path, $mo_link );
			}
			if (file_exists($po_file_path)) {
				$po_link = $theme_lang_dir_path . $locale . '.po';
				symlink( $po_file_path, $po_link );
			}
		}
	}
}