While the SDK that is generated by WorkflowFirst is in C# , you can actually use the SDK with any .Net language. To do this you will need to compile the SDK into a DLL first, and then reference that DLL along with the WorkflowFirst Connectivity DLL pack in your .Net project. Any language can be used, including Visual Basic.Net, Pascal, C++, Fortran, Java, LISP, JavaScript, Modula-2, Perl, PHP, SmallTalk, Prolog, COBOL, FORTRAN and TCL. 

To compile the SDK down to a DLL, you can use the following command from the folder where the WorkflowFirst Connectivity DLL pack and SDK .CS file resides: 

\windows\microsoft.net\Framework\v2.0.50727\csc /out:sdk.dll /t:library /o+ sdk.cs /reference: saxdaapi1.dll, xaml.dll, radeo.dll, celiumapp.dll
 
This assumes that Windows is installed in the \Windows folder, and that the SDK was saved as a .CS file, and that you are using Microsoft .Net version 2. Change the parameters to fit your precise configuration. 

This then creates a SDK.DLL file that you can reference in any .Net language.