The main languages spoken are Dzongka and English.
The local currency is the Ngultrum (equal to Indian Rupee). Credit cards are accepted at some hotels but not all of them, so it is safer to carry some local currency. If you need to change currency in Bhutan, you can exchange it at the airport upon arrival (counter located right outside the exit terminal towards their left hand side) or at a local bank. There are ATM's, but they are unreliable.
Only global cell phones will work and only in the bigger towns. You may want to consider renting one or getting a satellite phone.