In the literal sense, Mensch translates to mean ‘Human’. However; it is not always used in this way alone. It can also be an interjection in conversation, especially when something catches you off guard.
It has also been adopted in day-to-day speech as a way of expressing surprise at a situation, positive or negative. An example in English would be ‘Man!’ or ‘Oh man!’.
If you run into someone you have not seen for years on the street, you could say, for example, “Mensch, ewig nicht gesehen!” (“Oh man, I haven't seen you in forever).
It is often used to show dissatisfaction with someone or a situation. Let's say say that you hop on the U-Bahn to work and realize that you forgot to buy a ticket. At that very moment, you see ticket inspectors closing in from both sides as the doors close. 'Mensch!’
Alternatively, you can use it alongside a name to express disapproval or simply surprise towards something that the named individual may have done.
For example, when you get home from a long hard day at work or university and your flatmate Max has committed the very serious crime of eating the last brownie you were thinking about the whole way home. ‘Mensch, Max!’
In this scenario Mensch is combined with the name of the person who made the mistake. It is a succinct and direct way of getting your dissatisfaction with them across.
Mensch, Max! Was hast du denn gemacht?
Man, Max! What have you done?
Mensch! Der Kontrolleur wird mir eine Geldstrafe aufbrummen!
Oh man! The ticket inspector will fine me!