A main problem for electronic commerce, particularly for business-to-business applications, lies in the need for the involved information systems to meaningfully exchange information. Domain-specific standards may be used to define the semantics of common terms. However, in practice it is not easy to find those domain-specific standards that are detailed and stable enough to allow for real interoperability. Therefore, we propose an architecture that allows for incremental construction of a shared repository including a multilingual thesaurus, which is used in a business communication language. Communicating information systems then refer to the common thesaurus while exchanging messages. Our emphasis is be on separating semantics (in the thesaurus) and syntax (in XML). Therefore, our extensibility is not only that of XML, but also the extensibility of the semantics that is modeled in the shared repository. The business communication language XLBC is presented and how it can be used in electronic commerce applications. XLBC message patterns and conversation protocols are stored in the shared repository as well.