Change logo image condition based on url

if (strpos($_SERVER["REQUEST_URI"], "www.radarhh.com/stage/fr/") !== 0):

The $_SERVER["REQUEST_URI"] typically doesn’t contain the domain name. And you should be checking if it is zero, since strpos returns the position of the string, or false if it’s not in the string.

Try this instead:

if ( strpos( $_SERVER['REQUEST_URI'], '/stage/fr/' ) === 0 ) :

Also, you have this image:

<img src="https://wordpress.stackexchange.com/questions/165546/RadarHeadhuntersFR.png"> 

That should be a fully qualified URL, not starting with a “..” because that .. is relative to the URL, which might be different than the URL you’re expecting.

Note: There are cases where $_SERVER["REQUEST_URI"] can contain the full URL, however in such a case, you would want to check against the actual full URL, with the http and everything. Not against just the partial URL.