I'm not even talking about the little languages that just come in and out of fashion all the time.
If you know c++ you could basically roll right into C#, Java, PHP, etc in under a week and be doing real work in them. Maybe you wouldn't have the intricacies down immediately but you could be doing production code level work very quickly.
Learning a new language, say C++ -> Java is WAY more than just learning the syntax.
There are best practices for solving certain kinds of problems in each language that differ between language.
Sure, there is a high level similarity between the solutions (hey, its just the observer pattern, right?), but there are also likely (IME) to be many critical differences in the details.
You could probably write C++ code in C#, Java and PHP, but it would be hideous. I've seen Fortran written in C, and Pascal written in C++, and C written in Java, and it's generally an unholy mess.
If you know c++ you could basically roll right into C#, Java, PHP, etc in under a week and be doing real work in them. Maybe you wouldn't have the intricacies down immediately but you could be doing production code level work very quickly.