Language setting fixes:

- switch 'en_US' to 'en', fixes the "admin panel switches to Arabic" bug
- tweak setting descriptions to clarify that most of the time we'll be using browser language
- add a backend switch to disable language detection (should this be exposed to ui?)

lib/util.php

@@ -105,11 +105,13 @@ function common_language()
 
     // Otherwise, find the best match for the languages requested by the
     // user's browser...
-    $httplang = isset($_SERVER['HTTP_ACCEPT_LANGUAGE']) ? $_SERVER['HTTP_ACCEPT_LANGUAGE'] : null;
-    if (!empty($httplang)) {
-        $language = client_prefered_language($httplang);
-        if ($language)
-          return $language;
+    if (common_config('site', 'langdetect')) {
+        $httplang = isset($_SERVER['HTTP_ACCEPT_LANGUAGE']) ? $_SERVER['HTTP_ACCEPT_LANGUAGE'] : null;
+        if (!empty($httplang)) {
+            $language = client_prefered_language($httplang);
+            if ($language)
+              return $language;
+        }
     }
 
     // Finally, if none of the above worked, use the site's default...