They've added information to the Tips and Notes:

There are two forms of 'you' in Cymraeg, similar to other modern languages, being chi and ti. Chi is used with a group and people to whom you'd like to show respect. This is taught in this unit and ti later on. The form dych chi will change its meaning to you (are/do) to are/do you? when used with a question, much like in English.

That would mean "Pwy dych chi?" is for plural or formal and "Pwy wyt ti?" is for singular or informal. French does the same thing.