diff --git a/common.php b/common.php
index 333d2e821..d6806c4a6 100644
--- a/common.php
+++ b/common.php
@@ -350,6 +350,18 @@ function array_deep(&$var, $fn, $one_dimensional = false, $array_only = false, $
     }
 }
 
+function array_deep_merge($base, $overlay)
+{
+    foreach ($overlay as $key => $value) {
+        if (is_array($value) && isset($base[$key]) && is_array($base[$key])) {
+            $base[$key] = array_deep_merge($base[$key], $value);
+        } else {
+            $base[$key] = $value;
+        }
+    }
+    return $base;
+}
+
 /**
  * Hide BB_PATH
  *
diff --git a/src/Legacy/Common/User.php b/src/Legacy/Common/User.php
index b1a6a6713..04104fe6f 100644
--- a/src/Legacy/Common/User.php
+++ b/src/Legacy/Common/User.php
@@ -632,7 +632,7 @@ class User
         global $lang;
         require(LANG_DIR . 'main.php');
         setlocale(LC_ALL, $bb_cfg['lang'][$this->data['user_lang']]['locale'] ?? 'en_US.UTF-8');
-        $lang += $source_lang;
+        $lang = array_deep_merge($source_lang, $lang);
 
         $theme = setup_style();
         $DeltaTime = new DateDelta();