Agreed, the biggest problem of such project is where is the boundary. Once you implemented some fundamental features in language A, others will be forced to use

Agreed, the biggest problem of such project is where is the boundary. Once you implemented some fundamental features in language A, others will be forced to use that language for succeeding jobs. Even if you have already implemented some functions in language B, due to the limitation of integration, error handling or some other more important reasons, you still need to choose language A. Maybe after a lot of discussion, a solution can be finally reached. However, the effort consumed, or like now, causing personnel turnover, will significantly increase the cost of project. IMO if there is no extremely important reason, do not mix multiple languages within one project. If must, define some boundaries at the initial stage. Any attempt to change these boundaries should require agreement from everyone involved.