WPML Flags in Top Menu

1. from WPML > "Language switcher options" section
2. "Language switcher in the WP Menu" sub-section > tick the box "Display the language switcher in the WP Menu" and choose your mainmenu
3. "Language switcher style" sub-section > choose "Horizontal"
4. "What to include in the language switcher" sub-section > choose "Flag"

Then add CSS like this :

1. From Appearance > Themes Options > General (bottom Custom CSS) add this

li.menu-item-language img {border:1px solid #e7e7e7; } 
@media screen and (min-width: 750px) {
	li.menu-item-language, li.menu-item-language a {
		margin:6px 0 0 0 !important; padding:0 2px 18px 0 !important;
		} 
	li.menu-item-language-current a {
		margin:6px 0 0 50px !important }  
}  
@media screen and (max-width: 751px) { 
	li.menu-item-language, li.menu-item-language a {
		display:inline !important;
	}
}

Huge thanks for Julien for posting a solution!