I) English (noun) = The English language.

(adjective) Relating to England, or its people or language.

• English customs demand such polished manners.

II) The English = English people, the people of England.

• The English are among the world’s most cosmopolitan and well-travelled people.

III) Speak English – i.e., you know and can speak English.

• Do you speak English?
• My parents do not speak English.

IV) Speak in English = use English on a particular occasion.

• When I went to Chennai I had to speak in English as I didn’t know Tamil well.
• When the tourists came to Kovalam the guide spoke to them in English.