Question

What language, C or C++, is best suited for writing a native library once and using it in different languages with language bindings (for example, using JNI or Ruby’s C extensions), and why?

Answer 1

C, because

1. a program written in C++ can use a C library, the other way around is a whole lot trickier – any construct available in C++ but not in C cannot be exported to a C program, which risks to make your export complicated.
2. A C library will have less dependencies. C++ libraries will quite often depend on libstdc++, which is that big that for example on embedded systems or more generally on systems low on resources it won’t always be available.