Some blockchains have set interfaces for java, python , js for the convenience of the Dapp developers.
However almost all blockchains provide REST api set to interact with the blockchain.
A developer who want to cover multiple blockchains can deal with all of them in a uniform way by collecting and calling the specific api of each of the blockchains.
For more details on what a Dapp really is please check this article -
What a Dapp really is