Regardless of language you speak, you have grown up understanding the need for using formal language within the situations that warrant that is best it. Those situations being those that either circle around a serious subject or event, or involve individuals who we have no idea well.
Informal language, on the other hand, is much more commonly employed in the situations or scenarios where we are more stimulating and can often involve people that we know on a more personal level.
The utilization of formal language is much more prevalent as soon as we write. Informal language is observed more when we speak. Having said that, there are times when writing can be less formal.