Q:

Onclick gets fired without clicking

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...

A:

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');
};

A:

cause you must use something like that
en.onclick=function(){
  setCookie('english');
}

A:

Because you're invoking the method setCookie(...).  Try this:
en.onclick = setCookie;

With the brackets you're invoking the method; without you're handing it as an object. 
Or try this:
en.onclick = function() { setCookie('english') };

Here we create a new function which calls setCookie with the appropriate argument.