Why do you use the preposition ''in'' in the title of your article?

tl;dr

Summary:

To speak English = in general 

To speak in English = on a specific occasion

Ludmila, both are correct (both with and without "in").  It depends on the context and the implied meaning. I am trying to figure out the rule for you. Having thought about it, here is what I think:

If I say "do you speak English?" it means in general. I am asking about your general ability. I want to know whether you can speak English or not (in general). 

If we add the word "in", we are talking about a specific occasion (not in general). So if I say "do you speak in English?.........." it sounds like an unfinished sentence. I would have to add a specified day or time, and say, for example "do you speak in English on Thursdays with your teacher?" 

So if the title were to say "you speak English" it would mean in general.

However, since the writer has used the word "when", we are now talking about a specific occasion ("when" is a word that refers to a specific time).

So we would use the preposition "in," and say "when you speak in English" because we're talking about a specific occasion.

That's my theory. I hope it is right. 

@Jack Davidson,

Thanks, it is a very nice article. I notice that some small talks are boring in real life. I think if you don't want make such small talks boring, you should have your own opinions, disagree or agree with some other people. At meantime, you need to accept different opinions. When I speak in English, the main reason to make such talks boring is that I have less vocabulary so that I cannot express my thoughts in details, such conversations cannot  move on.

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Questions are supposed to be followed by question marks, not by full stops.

• Have you been here before.  
• Do you like calamari.