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

3. Open an app without a visible Settings menu such as Spectacle or Dolphin with its menubar hidden

4. Look in the help menu to make sure the action is shown there

BUG: 177856 FIXED-IN: 5.72

This is how it will look, in conjunction with kconfig!4 (merged) and kconfigwidgets!7 (merged):

cc @teams/vdg