Move "Switch Application Language" to Settings menu

This is where it logically belongs. Applications with only a Help menu
and no settings menu will still get it displayed in there due to some
fancy code in khelpmenu.cpp, so there are no cases where the menu item
will become completely inaccessible.

Test Plan:
1. Open Okular, Konsole, or Dolphin with its menu bar shown
2. Look in Settings and Help menus to make sure the action is in the
   Settings menu

1. Open an app without a visible Settings menu such as Spectacle or
   Dolphin with its menubar hidden
2. Look in the help menu to make sure the action is shown there

BUG: 177856
FIXED-IN: 5.72