x11/setxkbmap and FreeBSD documentation book.
To be more clarified... Read book about localization and internationalization. There generic information about configuring locales, languages and, for your topic, keyboard layouts and keys to switch between them. Than about available Desktop environments (of course after installing Xorg). That mentioned keyboard settings, that you may (or may not) put into Xorg config file, allows you to switch layouts. But every desktop environment (KDE, Gnome, Cinnamon, MATE, XFCE, LXDE) have it's own settings manager with it's part for keyboard settings. Like in windows. Most of them, by default, will use your settings for Xorg, if you write them. But in any case it's good to look there and set up that settings again for that specific DE. And
finally, about switching layouts by mouse, every DE has it's own app to display layout in tray (some installed and started by default, some, like in XFCE, if I remember it correctly, should be installed and added to panel manually, xfce keyboard indicator), that allows to switch keyboard layout by mouse.
