diff --git a/public/main/inc/lib/api.lib.php b/public/main/inc/lib/api.lib.php
index 3186448d22d..511c3407a47 100644
--- a/public/main/inc/lib/api.lib.php
+++ b/public/main/inc/lib/api.lib.php
@@ -3681,6 +3681,71 @@ function api_get_languages()
     return $languages;
 }
 
+/**
+ * Returns the platform default language isocode.
+ * Tries 'language.platform_language' (isocode). If empty, falls back to 'platformLanguage' (english_name) and resolves to isocode.
+ */
+function api_get_platform_default_isocode(): ?string
+{
+    // Preferred: isocode stored under 'language.platform_language'
+    $iso = trim((string) api_get_setting('language.platform_language'));
+    if ($iso !== '') {
+        return $iso;
+    }
+
+    // Fallback: older config storing english_name
+    $englishName = trim((string) api_get_setting('platformLanguage'));
+    if ($englishName === '') {
+        return null;
+    }
+
+    $langTable = Database::get_main_table(TABLE_MAIN_LANGUAGE);
+    $safe = Database::escape_string($englishName);
+    $sql  = "SELECT isocode FROM $langTable WHERE english_name = '$safe' LIMIT 1";
+    $res  = Database::query($sql);
+    if ($row = Database::fetch_assoc($res)) {
+        return $row['isocode'];
+    }
+
+    return null;
+}
+
+/**
+ * Returns available languages (available=1) plus the platform default language even if not available.
+ * Ensures the platform default never disappears from dropdowns.
+ *
+ * @return array<string,string> isocode => original_name
+ */
+function api_get_languages_with_platform_default(): array
+{
+    $langTable = Database::get_main_table(TABLE_MAIN_LANGUAGE);
+
+    // Standard available languages
+    $sql = "SELECT isocode, original_name FROM $langTable WHERE available = '1' ORDER BY original_name ASC";
+    $res = Database::query($sql);
+
+    $languages = [];
+    while ($row = Database::fetch_assoc($res)) {
+        $languages[$row['isocode']] = $row['original_name'];
+    }
+
+    // Ensure platform default is present even if not available
+    $defaultIso = api_get_platform_default_isocode();
+    if ($defaultIso && !isset($languages[$defaultIso])) {
+        $safe   = Database::escape_string($defaultIso);
+        $sqlDef = "SELECT isocode, original_name FROM $langTable WHERE isocode = '$safe' LIMIT 1";
+        $resDef = Database::query($sqlDef);
+        if ($row = Database::fetch_assoc($resDef)) {
+            $languages[$row['isocode']] = $row['original_name'];
+        }
+    }
+
+    // Optional: stable, human-friendly order
+    asort($languages, SORT_NATURAL | SORT_FLAG_CASE);
+
+    return $languages;
+}
+
 /**
  * Returns the id (the database id) of a language.
  *
diff --git a/public/main/inc/lib/formvalidator/Element/SelectLanguage.php b/public/main/inc/lib/formvalidator/Element/SelectLanguage.php
index c81e61538fe..45e881a10f1 100644
--- a/public/main/inc/lib/formvalidator/Element/SelectLanguage.php
+++ b/public/main/inc/lib/formvalidator/Element/SelectLanguage.php
@@ -16,20 +16,19 @@ public function __construct(
     ) {
         parent::__construct($elementName, $elementLabel, $options, $attributes);
 
-        $default = $attributes['set_custom_default'] ?? false;
-        if (!empty($default)) {
-            $defaultValue = $default;
-        } else {
-            $defaultValue = api_get_setting('platformLanguage');
-        }
-        // Get all languages
-        $languages = api_get_languages();
+        // Prefer a custom default if provided; otherwise use platform default isocode
+        $customDefault = $attributes['set_custom_default'] ?? false;
+        $defaultValue = !empty($customDefault) ? $customDefault : api_get_platform_default_isocode();
+
+        // Fetch languages available + platform default (even if not available)
+        $languages = api_get_languages_with_platform_default();
+
         foreach ($languages as $iso => $name) {
-            $attributes = [];
-            if ($iso === $defaultValue) {
-                $attributes = ['selected' => 'selected'];
+            $optAttrs = [];
+            if ($defaultValue && $iso === $defaultValue) {
+                $optAttrs = ['selected' => 'selected'];
             }
-            $this->addOption($name, $iso, $attributes);
+            $this->addOption($name, $iso, $optAttrs);
         }
     }
 }