The main deciding factor whether to use formal or informal language is your

age

audience

experience

grammar

you would not use formal language with

a policeman

your teacher

your little brother

the president

identify the example of informal language.

ryan was a beast in the game.

how do you like your tea, madam?

i experienced an issue yesterday.

billy bob is an exceptional talent.

formal language is used to communicate and acknowledge

kindness

attitude

respect

opinion