Create a uniform language for the empire

Allow different languages to co-exist

Adopt the new language as your own.

Put them to death

Enslave them

Allow them to retain power if they swear loyalty

It depends

Allow existing currency to continue

Create one currency for the empire

Do away with all forms of currency

Outlaw all religion

Allow existing practices to continue

Install your own religion, making all others illegal.

Destroy them

Allow them to stand

Outlaw and destroy it 

Respect and use it

Choose what's useful to you and destroy the rest.