Talk to a child anytime he is in your care, or you are anywhere near him. Even very young children, who can't talk back yet, use the process of listening and understanding, so talking to a child at every opportunity is a good way to help him discover and develop language. Don't ignore children, especially when you are talking or when they begin to talk back.
Ask children questions and listen to their responses. When a child volunteers information, listen to that information, and then talk back to her to show her that you have listened and that you understand what she is saying. This helps promote language development of all sorts.
Ask a child questions that he can answer, such as "Which is the red one" or "How many are there?" to get him talking back and to encourage him to develop language skills of his own.
Encourage looking at books, even before children can read. The act of looking at a book, seeing where the words are and understanding the concept of the book telling a story is one step in further language development.