It's possible, but you can't use the SmugMug menu. You will have to create a menu using a HTML/CSS block. You will also have to design your flag.
So I can not do it Thank you for your answer.
You can, but you'll have to use a HTML/CSS block. I messed around last night and came up with this: https://gallery.imagesinthebackcountry.com/Smugmug-customization/Custom-Menu-with-Image
I haven't finished writing the tutorial, but the code I have up there now will work.
Unrelated, but I noticed you live in Budapest. My mother's father used to visit Budapest years ago (he has since passed) because he had a brother that stilled lived there. Looks like a nice place!
Thank you very much!
and yes, Budapest is a nice place!