If you just install a fresh Centos 7 and you get that strange message every time when you are do something with yum.
But with rpm there is even more problems.
Failed to set locale, defaulting to C
This is a quick way to change the locale settings in CentOS 7.
You just need to create a new configuration file like:
In this new file, enter the needed parameters like mine in this case:
Save the file, and load its parameters using:
After you can log off and log on and test again yum update.