Things in my website's header are overlapping when I shrink my browser window

I have a problem with the page menu and the language menu in the header when I shrink my browser window. The problem is that they are overlapping when I shrink the width of the browser window.

The page menu goes automatically to the left when I shrink the browser window. In addition, a part of the page menu is missing when the browser window’s width is too small.

How can I fix this problem?

Before shrinking:

After shrinking: