You can download the source code to various versions from the download page. For those with svn,
svn co https://yammy.svn.sourceforge.net/svnroot/yammy yammy
Different versions of Yammy were built using different tools. For Yammy 0.4 and lower, you will need a copy of Dev-C++. Yammy 0.5-0.7 were compiled in .NET 1.0 using SharpDevelop. Version 0.9 onwards use Visual C# Express Edition.
Yammy wouldn't be the same if it didn't support multiple languages. Thanks to the translators, Yammy is available in 15 languages (and growing). Would you like to translate Yammy too?