din't is easily a sticky key or a fast-typing error. It happens to any of us and doesn't necessarily say that the person has a separate first language.
Furthermore, even though it's good form to read over a message you're sending and remove the typoes, grammar-os and other such issues, it doesn't always happen :)
Furthermore, even though it's good form to read over a message you're sending and remove the typoes, grammar-os and other such issues, it doesn't always happen :)