Centralize language loading with English fallback (#361)

GameEngine/Session.php

@@ -60,12 +60,8 @@ include_once("Form.php");
 include_once("Generator.php");
 include_once("Multisort.php");
 include_once("Ranking.php");
-include_once("Lang/" . LANG . ".php");
-// en.php is an idempotent fallback (tz_def only defines missing keys); loading
-// it after the player language guarantees every interface constant is defined,
-// so a key missing from a translation degrades to English instead of a PHP 8.3
-// "undefined constant" fatal (blank page). See issue #189.
-include_once("Lang/en.php");
+require_once __DIR__ . "/Lang/loader.php";
+tz_load_language(LANG);
 include_once("Logging.php");
 include_once("Message.php");
 include_once("Alliance.php");
@@ -585,4 +581,4 @@ if (!empty($_SESSION['id_user'])) {
     $message = new Message;
     $user = new User((int)$_SESSION['id_user'], $database);
 }
-?>
+?>