Where can I find the C# 5 language specification?

If you have Visual Studio 2012 installed, you will find specification somewhere there:

c:\Program Files (x86)\Microsoft Visual Studio 11.0\VC#\Specifications\1033\CSharp Language Specification.docx

similar with VS2013:

c:\Program Files (x86)\Microsoft Visual Studio 12.0\VC#\Specifications\1033\CSharp Language Specification.docx

VS2015:

c:\Program Files (x86)\Microsoft Visual Studio 14.0\VC#\Specifications\1033\CSharp Language Specification.docx

It was originally unavailable online but since June 2013 it is available for download from Microsoft.