Centralize language loading with English fallback (#361)

GameEngine/BBCode.php

@@ -21,7 +21,8 @@
 #################################################################################
 
 include_once("config.php");
-include_once("Lang/" . LANG . ".php");
+require_once __DIR__ . "/Lang/loader.php";
+tz_load_language(LANG);
 
 /**
  * Ensure input exists (avoid undefined variable issues)
@@ -207,4 +208,4 @@ $bbcoded = preg_replace_callback(
 $bbcoded = preg_replace('/\[message\]/', '', $bbcoded);
 $bbcoded = preg_replace('/\[\/message\]/', '', $bbcoded);
 
-?>
+?>

GameEngine/Lang/loader.php

@@ -0,0 +1,53 @@
+<?php
+
+/**
+ * Load one interface language and fill its gaps from English.
+ *
+ * Language files define constants through tz_def(), so loading the selected
+ * locale first preserves translated constants while English defines only the
+ * missing ones. Their legacy $lang arrays need an explicit merge because those
+ * assignments are not guarded.
+ */
+if (!function_exists('tz_language_array_from_file')) {
+    function tz_language_array_from_file($file)
+    {
+        $lang = [];
+        require $file;
+
+        return is_array($lang) ? $lang : [];
+    }
+}
+
+if (!function_exists('tz_load_language')) {
+    function tz_load_language($language, $languageDirectory = __DIR__)
+    {
+        global $lang;
+
+        $language = strtolower(trim((string) $language));
+        if (!preg_match('/\A[a-z_]+\z/', $language)) {
+            $language = 'en';
+        }
+
+        $languageDirectory = rtrim($languageDirectory, '/\\');
+        $englishFile = $languageDirectory . '/en.php';
+        if (!is_file($englishFile)) {
+            throw new RuntimeException('English language fallback not found: ' . $englishFile);
+        }
+
+        $selectedFile = $languageDirectory . '/' . $language . '.php';
+        if (!is_file($selectedFile)) {
+            $selectedFile = $englishFile;
+        }
+
+        $localized = tz_language_array_from_file($selectedFile);
+        if ($selectedFile === $englishFile) {
+            $lang = $localized;
+            return $lang;
+        }
+
+        $english = tz_language_array_from_file($englishFile);
+        $lang = array_replace_recursive($english, $localized);
+
+        return $lang;
+    }
+}

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);
 }
-?>
+?>