[Resuelto] Flags do not show in menu

Background of the issue:
I configured the Menu switcher as I did with several other customers. For some reason, the flags do not show in the menu. I disabled all irrelevant plugins to see if there was a plugin conflict. This had no result. Link to a page where the issue can be seen: enlace oculto

Symptoms:
The language flags do not show in the menu.

Questions:
Why are the language flags not showing in the menu?
Could there be another reason besides plugin conflict for the flags not displaying?