Traditionally the VSLangProjXX.dll files contain interfaces such as VBProjectPropertiesX, CSharpProjectConfigurationPropertiesY and VSLangProj90.dll contains them but it seems that it was forgotten to add properties to them:

BUG: Empty interfaces in VSLangProj90.dll
http://www.mztools.com/articles/2008/MZ2008018.aspx