I have always wondered why the iyor tense is used when asking if someone knows a language. We can say Et yersin, as if it's a permanent fact that you eat meat, but we don't say Fransizca bilirsin, which seems like a permanent fact about you, not a thing you're doing at this moment like geliyorsun.
It is just kind of an idiomatic thing. I wish I had a better answer :)
It is named ''anlam kayması'' (''semantic shift'') in Turkish. In this, we say something with ''-yor'' (particle of present continuous tense) but we actually doesn't mean that we do that at the moment. For instance: Fransızca bilirim. - I can speak (know) French. In this sentence, I asked that you know (speak) French or not. I asked it in present continuous but I meant in simple present. If I both said and meant in simple present tense: Fransızca bilirim/konuşabilirim.
Yarın İstanbul'a geliyor mu? - Will (s)he come to Istanbul tomorrow? I said in present continuous but I meant in future tense. If I both said and meant in future tense: Yarın İstanbul'a gelecek mi?
In summary, we use the present continuous tense for present, present continuous and future tense in Turkish. You don't have to use the particle of present continuous tense when you mean something in any tense out of present continuous.
In addition, there are different ways when you want to say that you can speak any language (with semantic shift):
Türkçe biliyorum. - I know Turkish. Türkçe konuşabilirim. - I can speak Turkish. Türkçe konuşuyorum. - I speak Turkish.
You can mean ''I am speaking Turkish.'' while saying ''Türkçe konuşuyorum.'', as well. You may think it's even a bit foolish sentence (because a Turk can understand that you're speaking Turkish). But don't think so, because, for example, our teachers warn us when we talk too much to each other during the lessons -although they had already warn us previously-. And they say ''Türkçe konuşuyorum, Arapça ya da Çince değil. Beni anlamıyor musunuz?'' They mean ''I am speaking Turkish, not Arabic nor Chinese. Don't you understand me?'' Also note we can use both ''konuşmak'' or ''konuşabilmek'' verbs when we want to say that we (can) speak any language. And we may use ''bilmek'' (to know)verb, as well, however we can't use ''bilebilmek'' (can know). Forwhy, yes ''-ebilmek'' (can) usage is used to talk about any ability, but you must have options more than one if you want to use this usage. For example, when you say that you can swim, it means you're able to swim and you have two options: to swim or not to swim. But ''to know'' verb is basically doesn't offer alternatives; you just know or not. But it has some exceptions. To illustrate, suppose that you have seen a question and you said your student to solve it. He/She isn't sure that (s)he can do it or not, and you say: ''Bence sen bu soruyu bilebilirsin.'' - I think you can know (answer) this question.
Don't worry, I'm a secondary school student in Turkey and ''semantic shift'' is a subject of Turkish even here. But it's not a hard subject because we speak this language since years. Maybe you can understand better if you practice speaking. I hope I didn't make you confused :( Hoşçakal! :)