Index: wp-includes/formatting.php
===================================================================
--- wp-includes/formatting.php	(revision 49708)
+++ wp-includes/formatting.php	(working copy)
@@ -1919,7 +1919,7 @@
 		// Used for locale-specific rules.
 		$locale = get_locale();
 
-		if ( in_array( $locale, array( 'de_DE', 'de_DE_formal', 'de_CH', 'de_CH_informal' ), true ) ) {
+		if ( in_array( $locale, array( 'de_AT', 'de_DE', 'de_DE_formal', 'de_CH', 'de_CH_informal' ), true ) ) {
 			$chars['Ä'] = 'Ae';
 			$chars['ä'] = 'ae';
 			$chars['Ö'] = 'Oe';