When you type: locale --help on the command line and your default locale is English then you get the description in English. That's correct. But you can switch the locale easily: LC_ALL=de_DE.utf8 locale --help LC_ALL=es_ES.utf8 locale --help LC_ALL=pl_PL.utf8 locale --help and you still get English despite the translation is correctly provided. It's really ironic that the locale command line utility which is meant to provide the locale information does not handle its own internationalization correctly. :-)
Sorry for the noise. This is not an upstream glibc bug, only Fedora packaging bug: it does not provide the libc.mo file unless the proper langpack file is installed. Filed downstream: https://bugzilla.redhat.com/show_bug.cgi?id=1624528