'To let' means 'to rent' in this context.

From the Cambridge dictionary.

let <em>verb</em> (RENT)
› mainly UK (US usually rent) to allow your house or land to be lived in or used by someone else in exchange for a regular payment:

So an alternative sentence would be 'have you heard that Netherfield Park has finally been rented out. The person owning the park has allowed someone to use it, in exchange for payment.

It is easy to understand, if a little formal. 'Let' is still quite often used as a verb in the UK. Nowadays it might be more natural to use the present perfect 'has been let' rather than 'is let' but who am I to argue with Jane Austen?

let verb (RENT)

mainly UK (US usually rent) to allow your house or land to be lived in or used by someone else in exchange for a regular payment

Netherfield Park is let means, that a person has finally decided to rent it for a given amount.  Make a regular payment of amount X to to the owner of the part in order to use it.

Oh, Chris! It was in the English book. ("Pride and Prejudice"):
"My dear Mr.Bennet, have you heard that Netherfield Park is let at last?"

I can't translate :-/

I'me not sure exactly what you're trying to say because it doesn't really make sense to say "park is let at last" ...