To enable language-level integration of multiple programming languages into a single programming framework, one or more domain specific languages (DSLs) are specified as incremental extensions to a host programming language. An application created in this framework may include program code written in the host language as well as DSL inset code that is written in any of the DSLs. The extensions to the host language are described in DSL specifications, which define how the DSL inset code is processed. At runtime, DSL inset code in the source code is detected and passed to a DSL inset processor for execution according to an appropriate DSL specification, whereas source code in the host language is executed by a host language processor.