Question

en.onclick = setCookie('english');

Why does this get fired without even clicking on it?

I have 3 flags that should set a cookie to their language when clicked wit the last one always gets fired right away…

Answer 1

Your code above evaluates setCookie('english') and puts its returned value (if any) into en.onclick. In order to assign it as an event handler, wrap it into a function:

en.onclick = function(){
   setCookie('english');
};