MDL-23037 Improved handling of the language preferred by remote users

Strips _utf8 suffix for remote users roaming from Moodle 1.x sites.
Checks that the preferred language is actually installed, uses the site
default otherwise.

auth/mnet/auth.php

@@ -254,6 +254,16 @@ function confirm_mnet_session($token, $remotepeer) {
         $remoteuser->auth = 'mnet';
         $remoteuser->wwwroot = $remotepeer->wwwroot;
 
+        // the user may roam from Moodle 1.x where lang has _utf8 suffix
+        // also, make sure that the lang is actually installed, otherwise set site default
+        $remoteuser->lang = clean_param(str_replace('_utf8', '', $remoteuser->lang), PARAM_LANG);
+        if (empty($remoteuser->lang)) {
+            if (!empty($CFG->lang)) {
+                $remoteuser->lang = $CFG->lang;
+            } else {
+                $remoteuser->lang = 'en';
+            }
+        }
         $firsttime = false;
 
         // get the local record for the remote user