Answer:
We use standard formal English when communicating with an unknown audience.
Less formal, colloquial English would be fine when communicating with people whom you know and who also know you.
Explanation:
Standard formal English is mostly used in academic writings, job applications, persuasive letters to authorities, business presentations within a company, or to some other company.
Less formal and colloquial English is used when we communicate with people whom we already know, and who also know us. For example communicating with a team lead, group discussion, business meetings within a company, talking to your employer with whom you have already worked for some time.
Below this less formal style comes informal style (though acceptable but not correct) which we use with our close friends, team mates, etc.
The lowest level is slang which is hardly acceptable for most of the people.
