Makes sense. One really can't have very many languages in which one is confidently fluent no matter what the topic. I think I may say I've gotten there with English, but after spending the last ten years reading and writing it for quite possibly an average of several hours a day, well, that's no wonder. It kind of depresses me, how even after such extreme amounts of time my spoken English at least could still be better (mostly in terms of being consistent with an accent I'm generally all over the place even within a single conversation). Makes one wonder how one can ever really attain native fluency in more than two or three foreign languages.

Yes, those can be entirely different things... and then some people noted that they were far better conversationally than written. I think I myself might say I speak French better than I write it, though that's mostly a matter of some grammar rules I'm just too lazy to learn properly, and I do read it fluently, so...

To read is very easy, yes, to listen to some extent, and to make myself understood after a fashion. But to speak and write correctly? When two languages are that close together, false friends become a major problem, and speakers of Dutch have a tendency of just not putting much effort into learning German, because we understand most of it already anyway. Of course I could learn German with far less effort than I've put into learning French or Spanish; but we're generally just not willing to put that much effort into it, unless we have some particular reason to need to speak/write perfect German.

As for Arabic, with the written language (MSA or Classic Arabic), it's mostly a matter of its enormous vocabulary - I knew more of it a few years back when I was still studying it in uni, obviously, maintaining that level of reading/writing is a challenge. And then the spoken language, well, I've just never had much practice in speaking and listening, even in MSA. And hardly any in the spoken varieties, I know some words and some general rules of the spoken languages, but that's about it.

A week or two ago, a friend of mine who studied Arabic with me in uni and myself went to see some play in Moroccan Arabic - expecting to be hard-pressed to follow it, alright. What we had not exactly expected was that we'd understand essentially nothing at all - perhaps a word every five sentences, and the handful of sentences that were more or less straightforward MSA.