What language should the API be written in?

We want to implement the API, we have a database located on a central server, and a network of many computers. On these computers in the future several local programs will be developed using different programming languages, some of them in java, some in perl, C ++, ... etc.

These local programs must have access to the API functions and interact with the database. So what language should the API be written in? so that it will be attached to other languages. Is there any specific architecture that needs to be implemented? Is there any link that would provide useful information about this?