Greek layout not displaying correctly.

On the latest Ubuntu version, with three different languages installed (English, French and ancient Greek (m17n)), only the Latin alphabet displays correctly. This means that Onboard functions as expected when the language is either English or French, displaying a qwerty or azerty layout as expected.
However, when setting the language to ancient Greek, Onboard's layout is not updated, displaying the Latin alphabet (either qwerty or azerty depending on the previously used language) despite generating Greek letters when pressing the corresponding key (eg: alpha on a). I would like to display the Greek alphabet when the language is set to ancient Greek.
I suppose there is something wrong with the configuration, I even tried to include the layouts from an old Ubuntu computer where the Greek alphabet is displayed correctly, to no avail.
Am I missing something?