While the current release just includes compilers and APIs for C++, Java, and Python, the compiler code is designed so that it's easy to add support for other languages. There are several ongoing projects to add new language implementations to Protocol Buffers, including C, C#, Haskell, Perl, Ruby, and more.
For a list of links to projects we know about, see the third-party add-ons wiki page.
Compiler Plugins
As of version 2.3.0 (January 2010), protoc, the Protocol Buffers
Compiler, can be extended to support new languages via plugins. A plugin is
just a program which reads a CodeGeneratorRequest protocol buffer
from standard input and then writes a CodeGeneratorResponse
protocol buffer to standard output. These message types are defined in
plugin.proto.
We recommend that all third-party code generators be written as plugins, as this
allows all generators to provide a consistent interface and share a single
parser implementation.
Additionally, plugins are able to insert code into the files generated by
other code generators. See the comments about "insertion points" in
plugin.proto
for more on this. This could be used, for example, to write a plugin which
generates RPC service code that is tailored for a particular RPC system. See
the documentation for the generated code in each language to find out what
insertion points they provide.