Index: includes/Linker.php
===================================================================
--- includes/Linker.php	(revision 88299)
+++ includes/Linker.php	(working copy)
@@ -35,6 +35,8 @@
 	 * @param $unused String: unused
 	 * @param $class String: the contents of the class attribute; if an empty
 	 *   string is passed, which is the default value, defaults to 'external'.
+	 *
+	 * @todo this function seems to be unused now... deprecate?
 	 */
 	static function getInterwikiLinkAttributes( $title, $unused = null, $class = 'external' ) {
 		global $wgContLang;
@@ -45,7 +47,13 @@
 		$title = $wgContLang->checkTitleEncoding( $title );
 		$title = preg_replace( '/[\\x00-\\x1f]/', ' ', $title );
 
-		return self::getLinkAttributesInternal( $title, $class );
+		$attrs = self::getLinkAttributesInternal( $title, $class );
+
+		if ( $wgContLang->getLanguageName( $iw ) ) {
+			$attrs .= ' hreflang="' . htmlspecialchars( strtolower( $iw ) ) . '"';
+		}
+
+		return $attrs;
 	}
 
 	/**
@@ -273,7 +281,15 @@
 			}
 
 			if ( $target->isExternal() ) {
+				global $wgContLang;
+
 				$classes[] = 'extiw';
+				$iw = strtolower( $target->getInterwiki() );
+				if ( $wgContLang->getLanguageName( $iw ) ) {
+					// Note that we cannot guess if the text of the link is in
+					// that language reliably, so we only set hreflang, not lang.
+					$defaults['hreflang'] = $iw;
+				}
 			}
 
 			if ( !in_array( 'broken', $options ) ) { # Avoid useless calls to LinkCache (see r50387)
Index: includes/SkinTemplate.php
===================================================================
--- includes/SkinTemplate.php	(revision 88299)
+++ includes/SkinTemplate.php	(working copy)
@@ -476,7 +476,9 @@
 						'text' => ( $wgContLang->getLanguageName( $nt->getInterwiki() ) != '' ?
 									$wgContLang->getLanguageName( $nt->getInterwiki() ) : $l ),
 						'title' => $nt->getText(),
-						'class' => $class
+						'class' => $class,
+						'lang' => $nt->getInterwiki(),
+						'hreflang' => $nt->getInterwiki(),
 					);
 				}
 			}
@@ -1558,7 +1560,7 @@
 		}
 
 		$attrs = array();
-		foreach ( array( 'href', 'id', 'class', 'rel', 'type' ) as $attr ) {
+		foreach ( array( 'href', 'id', 'class', 'rel', 'type', 'lang', 'hreflang' ) as $attr ) {
 			if ( isset( $item[$attr] ) ) {
 				$attrs[$attr] = $item[$attr];
 			}
@@ -1611,7 +1613,7 @@
 			}
 		} else {
 			$link = array();
-			foreach ( array( 'text', 'msg', 'href', 'rel', 'type', 'tooltiponly' ) as $k ) {
+			foreach ( array( 'text', 'msg', 'href', 'rel', 'type', 'tooltiponly', 'lang', 'hreflang' ) as $k ) {
 				if ( isset( $item[$k] ) ) {
 					$link[$k] = $item[$k];
 				}