Different languages have different libraries with different functions that take different arguments and have different data structures. The tools you used with the old language might be different or required modification to work with the new language.

Maybe libraries they had written to solve problems in their domain have to be recreated.

It seems like switching from Delphi to Java or Go and changing the entire ecosystem would take a bit of time to get back to making the same productivity as with the toolset one has been using for a decade?