Hi,
1. Can you speak English?
2. Can you speak in English?
3. I want to learn how to speak English.
4. I want to learn how to speak in English.
Are the above sentences all correct? Yes.
Is there any difference in meaning if "in" is used?? Usually, 'in' is not used. It doesn't sound natural in any of these examples.
Let me comment by offering a couple of examples.
If I were in a foreign country, and I walked into a hotel, I might say to the man behind the counter, 'Can you speak English?'
Now consider a context where different languages are being discussed. You are German. Yous say something in Japanese. I might naturally say to you, 'Can you say that in English?'
Best wishes, Clive