Polyglot programming is the idea to use in a single project (application or script) multiple programming languages at the same time. "Use the right language for the job" is often referred as the main driver behind this movement. Productivity increase is the overarching goal, justified by disjoined qualities of special purpose programming languages.
Now lets assume the project survives the development, is successful and is going to be maintained for several years.
- How many developers are able to master several languages at the same time to maintain the system efficiently?
- How much time are you going to spend in meetings to discuss what the best language for the job actually is?
- How likely is it, that all the chosen languages remain popular and well maintained through the lifecycle of the application?
- How many IDEs are you going to need to maintain the whole system?
- How easy it is to create a Continuous Integration pipeline for all the languages, but one system at the same time
- Did the complexity really decreased and the maintainability increased?
- How interoperable are theses languages?
- What about debugging, profiling, bug fixing? Is a language mix in the same callstack easier to tackle?
GWT on the other hand, is still popular among Java backend developers, because they can build the frontend ...in Java.
Usually not the technology, rather than annoying client's requirements are the productivity break :-)