After some testing, it looks like the issue is how the Tk is handling the Shift key with the French keyboard mapping.
For example, the Bang shortcut is bound as:
bind all <$::modifier-Shift-Key-B> {menu_send %W bng}
bind all <$::modifier-Shift-Key-b> {menu_send %W bng}
and the menu item:
$mymenu add command -label [_ "Bang"] -accelerator "$accelerator+Shift+B" \
-command {menu_send $::focused_window bng}
With the English keyboard mapping, the $::modifier-Shift-Key-b fires and the menu item's command is ignored (expected behavior).
With the French keyboard mapping, the $::modifier-Shift-Key-B fires AND the menu item command fires. It seems as though the capital B doesn't get matched to the menu accelerator binding.
The German keyboard mapping behaves the same as the English one.
I can confirm seeing this with a French keyboard mapping and my system set to English.