What do you know about these? Say: - in what contries

In what countries people use English as their mother tongue. People use English as their mother tongue in England.

 In what countries they use English as the second important language. People use English as the second important language in India, Canada, Nigeria, Singapore.

Where in the world people are interested in learning English. People are interested in learning English all over the world.

 Why people think that English has become an international language. People think that English has become an international language because there are so many English speakers in the world.

What countries are English-speaking countries? English-speaking countries are the UK, the USA, India, Canada, and Australia.

 How much has English developed since the 16th century? English has developed a lot since the 16th century.

 What variants of English do you know in the modern world? There are British English and American English, Australian, Canadian, Indian. 

 What are the most important kinds of English in the 21st century? The most important kinds of English in the 21st century are British English and American English.

 If speakers of British English and American English can understand each other. Yes, they can. They can understand each other.

 If you know the words which mean the same in British and American English. Autumn is in British English and "fall is in American English.