Minimalist accordion implementation using vanilla JavaScript
Yes, you should use classList. Right now you overwrite the entire className attribute of the elements. It works, but it also means that you can never use any other class name on the elements. So it's just not a very precise or robust way of doing things; it only works as long as you don't add other classes. And if you do need to add another class to the elements at some point, your script will mess up because the comparison with "" will always fail, and then any and all class names will get summarily removed by your script.

Of course, classList isn't supported everywhere, so you may have to use a shim/polyfill. The nextElementSibling property isn't supported everywhere either, by the way (which, incidentally is why jQuery exists; because browser compatibility is not simple).

I also wonder why you use querySelector in one line, but elementsByClassName in the next line. In both cases you want to find element(s) with a given class name. Of course, in the first case, you want exactly 1 element, and in the other you want a collection. However, if you want 1 particular element, you should probably add an id attribute to that element - not a class - and then use the good old document.getElementById. In other words: Using a class selector and expecting it to only exist on one element is backwards. Classes are meant to be used on an arbitrary number of elements. If you want unique identification, use an id instead.

Also, please don't mix languages in your code. I know it's only two variable names, but especially because it's only two variable names there's no reason to switch language. The DOM API is in English, and even all your CSS classes are in English, so it's simpler and cleaner to just stick to that (by the way, I'm not a native English speaker myself, I just prefer code to be consistent.)

Lastly, I always recommend that you use brace-on-same-line style in JavaScript. There are some assumptions about style in JS interpreters, and brace-on-same-line is one.