When using RealURL in multi language environment sending an real 404 is sometimes not so easy. Some I just want to give you some hints:

First of all enable the pageNotFound_handling of Typo3.

• open the file typo3conf/localconf.php
• add these lines
  
$TYPO3_CONF_VARS["FE"]["pageNotFound_handling"] = 'http://www.my-domain.com/en/not-found.html';
$TYPO3_CONF_VARS["FE"]["pageNotFound_handling_statheader"] = 'HTTP/1.1 404 Not Found';


Now you need to check your RealURL config:

• Be sure you that you have not set postVarSet_failureMode OR set it to postVarSet_failureMode=''
• Also be sure you have defined 'noMatch'=>'bypass' for every single preVars section!

That’s all 🙂 Now you get the your wonderful 404 file not found page every time when a wrong page is hit.