In Fred Brook's book, The Mythical Man Month, there is a chapter on "Adding People to a Late Project Makes it Later". The reason given is the new people have to

In Fred Brook's book, The Mythical Man Month, there is a chapter on "Adding People to a Late Project Makes it Later". The reason given is the new people have to be brought up to speed (trained) on the project by people already working on a project. This is magnified if the project uses a different programming language, libraries, OS, database than other projects. Multiple programming languages: just say no.