The situation that is appropriate to write using informal language is a postcard to a friend.
The situation in which it is appropriate to write using informal language is when you are writing a postcard to a friend. In this case, informal language is acceptable because a postcard is typically used for casual communication with someone you have a personal relationship with. It allows you to express yourself in a more relaxed and conversational manner. However, it is important to note that even in informal writing, it is still important to maintain a level of respect and appropriateness in your language. For example, you would still want to avoid offensive or vulgar language.
