How can Bitcoin be used by a charity in the Congo to receive donations from people around the world paying in different currencies?
You customers can exhange any currency to Bitcoin and pay via Bitcoin with no fees. So, you need to support traditional payment methods and Bitcoin as well, as a new way of payment over Internet. It's the future of payment.
Some day, you will kick-off VISA and Paypal from your website ;)
You can only send bitcoins on the Bitcoin network, so in fact you cannot receive payments in other currencies.
You'd need someone to receive the payments, exchange them for bitcoins, and then send bitcoins instead. However, this will only be useful to you, if the charity can make use of Bitcoin to pay for their operation cost or can get local currency in the Congo in exchange for Bitcoin.
There is currently payment processors that accept Bitcoin payments and forward fiat currency to you, yet as far as I am aware, there aren't any payment processors that accept fiat payments and forward Bitcoin to you.
The bitcoin network can only send bitcoins. You would need to set up a wallet and have the wallet address posted on your website/poster etc. A person would send their bitcoins, earnt or transferred via an exchange to your wallet. You could then transfer them back to local currency or buy things in bitcoin with them.
You can use Ripple coin that support a lot of currencies with bitcoin. https://ripple.com/