When is it important to use formal rather than casual language in a speech?

So that your audience will trust you.

Explanation:

If you give a speech and use casual language (y'all, like, actually...) your audience will discredit anything you say. However, if you use formal language then people will think that you know what you are talking about, even if what you are saying is complete bs.

i would think yes.

explanation:

because we have every natianlity in our country which makes us so diverse and unique. that's why in a lot of cities there are places like little italy, koreatown, chinatown.  

could you go a bit more in depth with you question? it's quite hard to understand. as soon as you do that i will be glad to : )