If you haven't set the keyboard layout system-wide, you can specify it in Xfce 4 Settings Manager
.
1. Open Xfce 4 Settings Manager
Menu => Settings => Settings Manager
2. Click on the Keyboard
icon.
3. Switch to the Layout
tab.
4. Untick Use system defaults
5. Click the Add
button and choose the appropriate keymap from the list.
6. Select the active layout.
If you need to switch between different layouts, installing the Xfce panel keyboard plugin might be a more convenient option.
1. Install xfce4-xkb-plugin
from SlackBuilds
2. Right-click on the Xfce panel and choose Add new items
3. Select Keyboard
and click Add
4. The keyboard plugin should appear in your panel.
Note: Also you can set your keyboard layouts manually, by:
setxkbmap -option grp:switch,grp:alt_shift_toggle