Hello and thank you for this.
With 1.0.15, Sub level menu works well, thank you.
Please note (now) an issue on the top left part of the screen : we keep the menu in blue text after a vertical menu :).
To get this issue:
Get a screen with a horizontal menu,
Retract the width screen and get a hamburger icon menu ?
Click on the hamburger menu icon and get the list menu
Enlarge the screen to get a horizontal menu again.
A blue text menu appear at the top of the page.
Refresh the page, the blue text menu disappears.
Do not hesitate to have a look here : http://mistreetsurf.tulipas.fr/
(In English and to French thank to MultilingualPress extension).
Note I have fixed the menu with a CSS modification.
Thank you for your time and considération, José.