Learn Polish: Skąd jesteś? Czy znasz polski? - Where are you from? Do you know Polish?

In Polish, skąd is a single word that means 'from where'. You cannot translate English sentences word-for-word by leaving a word like 'from' at the very end. Therefore, Skąd jesteś? literally means 'From where are you?'.

Why is there no word for 'you' in either of these questions?

Both jesteś (you are) and znasz (you know) have the 'you' meaning built into their endings. In Polish, you usually drop the pronoun ty (you) unless you want to add special emphasis.

What is the purpose of the word Czy?

Czy is a question word used to start 'yes or no' questions. While English uses words like 'do' or 'are' to do this, Polish simply puts Czy at the beginning of a statement to turn it into a question.

I thought the word for 'to know' was wiedzieć. Why do we use znasz here?

Polish has two different verbs for 'to know'. You use wiedzieć for knowing facts, but you use znać (which znasz comes from) when you are familiar with a person, place, or a language. Since this is about knowing the Polish language, znasz is the correct word.