Came across a post by Justin Etheredge discussion whether changing between languages is just a matter of syntax.
Or, to pick a specific example, can a Java programmer quickly and easily learn to write C# code?
The answer is obviously "yes". Development is about a way of thinking and approaching problems, and given the similarity between Java and C#, a good Java developer should take a minimal amount of time to learn how to write functional code in C#. The biggest barrier is libraries, which are more different than the languages are.
The answer is equally as obviously "no". Sure, you can write functional code, but you will not be able to write idiomatic code. Like a high school senior with 4 years of French class on a trip to Paris, you can make yourself understood, but you aren't going to be mistaken as a native. You ask a question, somebody replies, "Ce ne sont pas vos oignons", and you just end thinking of soup.
So, yeah, you can write C# code, but it's going to be Java written in C#. Given the closeness of the languages, it may be sufficient, but you're going to force some refactoring on any idiomatic C# speakers who inherit your code.
It can be worse - when I first started writing in Perl, I wrote C code in Perl, which just doesn't work very well. And over time, I became at least functional, though perhaps not idiomatic in Perl (though, because of TMTOWTDI, it's hard to judge that in Perl).
However, if you can become idiomatic in multiple languages, your toolset broadens, and you become more useful in all your langauges.