It depends on what you mean by “testing”. As downstream, we don’t add our own tests to packages—if upstream has tests as part of their released code, we run these during the build process.
So, if you’ve run into a bug about a particular software not working as expected in different locales, this should be reported to upstream in the first place. They should then add tests to check for this in the future, and when they make a release, all of this will end up in the Fedora package.
On a distribution level, we have test days, but these only check a very small subset of the total Fedora package set—primarily the main release artefacts.