Activate face recognition in the new Google Photos I'm living in Europe and the new Google Photos app does not group the people in my photos in the search menu. I did some research and found out that this function is only available in USA. 

Any idea how to activate it in other countries?

As mentioned in the comments, the reason for the absence of this feature is legal issues, so you are responsible for using this work around ;-)
I just found the solution to this. All you need to do is:


Connect to a USA server via a VPN 
Clear the data and cache of the Google Photos app. 
Open the app again and activate the "Group similar faces" in the settings
Check back the search menu after a couple of minutes and the faces would be there. Google already did the recognition for you, but kept it for itself.
Disconnect from the VPN.