Is it sufficient to backup the private keys of my android Dogecoin Wallet? I installed the langhans Dogecoin Wallet on my android device and backed up my private keys.
Is it sufficient to backup the wallet to avert loss in case of loss of or damage to the phone?
The private keys are the critical thing and are in the wallet backup.  They're what is needed to transfer coins. If they become public (i.e. stolen), whoever has them can transfer your coins to themselves. If you lose/forget your private keys, you can't make transfers, the coins are worthless to you, effectively lost.

You can create multiple addresses for bookkeeping. For example you can create an address for client X and an address for client Y, your wallet will show how much each address has received.  Also, creating a new address for every transaction can somewhat anonymize the transactions.