In situations which demand a quality written piece of work with no slang and complex prose, such as in articles or formal speeches, the writer should use formal language.
Explanation:
Formal use of language is necessary when a writer is confronted with an assignment or a client that demand complex prose and use of formal language throughout. In some situations a writer might need to write an article to be published in a top journal, or write a formal speech which will be read to in front of top executives, in which case he should use formal language
Talking to a person you do not know, an email to a business representative, and similar would be situations where someone would have to use formal language.
