km-config only installs the keyboard into IBus. The step to “install the Input Source in Region & Language Settings.” is needed for it to be used in gnome-shell.
https://help.gnome.org/users/gnome-help/stable/keyboard-layouts.html.en is the Gnome instructions for adding an input source.
For your French example on a system installed as English which doesn’t have French language installed in
Manage installed languages on
Region & Language settings. First you install the
eurolatin keyboard in “Keyman Keyboards”. Then go to the
Region & Language settings. The input source will be in
Otherand it is called
French (EuroLatin (SIL))