Hi Nikolaus,
Thanks for reaching out.
Unfortunately, it’s not possible within the builder and it’s also configured within WPML. I recommend setting your language switcher to dropdown instead of list. That will make sure the flags are group together instead of flag for each menu item (separated items like other menu items).

The sizing of the flags are also imposed by WPML and it will always resize to 18x12 pixels, please check this https://wpml.org/forums/topic/custom-flag-being-resized/. I recommend contacting WPML for the changes you preferred with the switcher, the integration is applied through them. They offer a solution from that URL but I’m not sure if it works.
Thanks!