How to install a new keymap


I would like to be able to switch between two keymaps, so I can type in english, as well as in my native language. When I try to set the keymap, I get:

localectl set-keyumap el_GR
Failed to set keymap: Keymap el_GR is not installed.

Can someone please let me know how can I add a new keymap?

Thank you

I managed to install the keymap using the GUI.

Settings → Keyboard → Input Sources → Click the plus sign → More and select the desired language.

Thank you