diff --git a/src/MD_STD.php b/src/MD_STD.php
index 6c8517b..d81ea69 100644
--- a/src/MD_STD.php
+++ b/src/MD_STD.php
@@ -428,6 +428,12 @@ final class MD_STD {
         }
         else {
             $lang = self::lang_getfrombrowser($allowed_langs, $default_lang, "", false);
+
+            // If the user is a bot or has no user agent at all or one of curl's,
+            // setting a cookie usually makes little sense.
+            // On the other hand, setting the cookie prevents effective caching.
+            if (empty($_SERVER['HTTP_USER_AGENT']) || substr($_SERVER['HTTP_USER_AGENT'], 0, 5) === 'curl/') return $lang;
+
             if (!setcookie('__Host-lang', $lang, $cookie_options)) {
                 throw new Exception("Failed to set language");
             }