Description of problem:
when testing F24 Alpha for translations, we noticed dnf-plugins core and extra where almost not translated in french.

to have the context of strings to translate, we need at least to access help content

but for plugins, it's not easy to understand how to print help because plugins don't have a consistent behavior

typing "dnf" would result in printing help
  => but it doesn't tell we can type "dnf PLUGIN --help-cmd" to get more information

typing "dnf builddep" would show help, same as "dnf builddep --help-cmd"
 => perfect

typing "dnf copr" would show a help message not consistent with builddep and "dnf copr --help-cmd" would show the same and failed
 => almost perfect

typing "dnf repoquery" would directly output some result while "dnf repoquery --help-cmd" would pring a full help
 => imho, it's ok, but not consistent with dnf behavior

typing "dnf playground" would directly show some help while "dnf playground --help-cmd" would fail : "Erreur : Unknown subcommand --help-cmd."
 => imho, it's not ok and not consistent with dnf behavior

Version-Release number of selected component (if applicable):
dnf 1.1.7-2

How reproducible:
always on F23 & F24

Steps to Reproduce:
1. type dnf PLUGIN
2. type dnf PLUGIN --help-cmd
3.

Actual results:
often different

Expected results:
should at least print help when passing --help-cmd

Additional info:
I targeted F24 as we are in F24 development cycle