"We will wait until Matěj returns."

Translation: Počkáme, než se vrátí Matěj.

[SteveWildones]

What is the difference between než and až?

[VladaFu]

That is a difficult question as there are so many individually distinguished meanings in the dictionary (https://bara.ujc.cas.cz/psjc/search.php?hledej=Hledej&heslo=a%C5%BE https://bara.ujc.cas.cz/psjc/search.php?hledej=Hledej&heslo=ne%C5%BE ).

The meanings will differ in positive and negative sentences.

In this specific sentence you can use both.