Added Polish language support, thx Patryk Kubiak

includes/locale.php

@@ -72,6 +72,9 @@ if (empty($_SESSION['locale']) && strlen($_SERVER['HTTP_ACCEPT_LANGUAGE']) >= 2)
     case "el":
         $locale = "el_GR.UTF-8";
         break;
+    case "pl":
+        $locale = "pl_PL.UTF-8";
+        break;
     default:
         $locale = "en_GB.UTF-8";
         break;

includes/system.php

@@ -143,6 +143,7 @@ function DisplaySystem()
         'ja_JP.UTF-8' => '日本語 (Japanese)',
         'ko_KR.UTF-8' => '한국어 (Korean)',
         'nl_NL.UTF-8' => 'Nederlands',
+        'pl_PL.UTF-8' => 'Polskie',
         'pt_BR.UTF-8' => 'Português',
         'ru_RU.UTF-8' => 'Русский',
         'sv_SE.UTF-8' => 'Svenska',