When trying to install Performance Center 12.50 using the French version installer, the error message says: "The upgrade patch cannot be installed by the Windows installer service, because the program to be upgraded may be missing or the upgrade pathc may update a different version of the program........" Attached there is a screenshot with the error message.
Here the process to install the software from French package:
1) Run the PC installation by clicking Performance Center Server.
2) Run the PC Language Pack for French by clicking Serveur Performance Center. (you can't run this before you actually install the PC Server - thus the error you had)
3) (Just for PC Server)Make sure to have the IIS settings for culture for \Sites\Default Web Site\LoadTest site - Globalization .NET in French set to Francais (France) (fr-FR). And restart IIS.
Similar for the PC Host (except step 3).