Hamburger menu on mobile

hi all, I recently modified the header of my theme Extendable in order to get a multilingual menu.

I would like to have a responsive menu on the mobile version of the website (now it is just overlapping the line of the menu and is thus not working well). I tried to add additional CSS and to modify the html, but somehow I miss some crucial steps because I can’t make it work. Can somehone help??

The page I need help with: [log in to see the link]