Index: wp-includes/l10n.php
===================================================================
--- wp-includes/l10n.php	(revision 23380)
+++ wp-includes/l10n.php	(working copy)
@@ -550,4 +550,48 @@
 	}
 
 	return $languages;
+}
+
+/**
+ * Get the PHPMailer localization name that corresponds to a WordPress localization name
+ *
+ * PHPMailer's localizations naming scheme doesn't match WordPress', so this function helps to match them up. Typically, that just means
+ * converting to lowercase and using the first two characters of the WP name, but there are also exceptions that are handled. PHPMailer
+ * will check if the resulting filename actually exists, and default to English if it doesn't, so checking that is not necessary here.
+ * 
+ * @since 
+ *
+ * @param string $wp_lang A WordPress localization name. Defaults to the localization currently being used.
+ * @return string The corresponding PHPMailer localization name.
+ */
+function get_phpmailer_localization( $wp_lang = WPLANG ) {
+	$wp_lang = strtolower( $wp_lang );
+		
+	switch ( $wp_lang ) {
+		case 'cs_cz':
+			$phpmailer_lang = 'cz';
+		break;
+		
+		case 'da_dk':
+			$phpmailer_lang = 'dk';
+		break;
+		
+		case 'pt_br':
+			$phpmailer_lang = 'br';
+		break;
+		
+		case 'sv_se':
+			$phpmailer_lang = 'se';
+		break;
+		
+		case 'zh_ch':
+			$phpmailer_lang = 'zh_ch';
+		break;
+			
+		default:
+			$phpmailer_lang = substr( $wp_lang, 0, 2 );
+		break;
+	}
+	
+	return $phpmailer_lang;
 }
\ No newline at end of file
Index: wp-includes/pluggable.php
===================================================================
--- wp-includes/pluggable.php	(revision 23380)
+++ wp-includes/pluggable.php	(working copy)
@@ -225,6 +225,7 @@
 		require_once ABSPATH . WPINC . '/class-phpmailer.php';
 		require_once ABSPATH . WPINC . '/class-smtp.php';
 		$phpmailer = new PHPMailer( true );
+		$phpmailer->SetLanguage( get_phpmailer_localization(), apply_filters( 'mailer_language_directory', WP_CONTENT_DIR . '/languages/phpmailer/' ) );
 	}
 
 	// Headers