funny behaviour with changing languages

I experience some strange behaviour that I can not yet explain.

What I did:

1. Created a new file called safe.php
2. Changed the in the URL of the about page in the Navigation Menu to a new file (lets say safe.php)
3. in the .htaccess create a new rewrite rule that sends requests for safe.html to safe.php

So far everything is working fine but now ...

When I change the language on this page, the page url is safe.php?lang=en and not safe.html?lang=en (as specified in the Navigation Menu)

Also when this page is opened the subMenu is not displayed any more. When I manually change the url back to safe.html?lang=en, the subMenu is displayed.

My questions, where does the language check get the safe.php from, instead of the safe.html?