Visual Foxpro. The application originated in 1986 or so and the original language was dBase II, but moved to FoxBase in 1987, and FoxPro/Visual FoxPro over the next couple of decades.
I do as much in Python as I can justify, but I still have to work in VFP quite a bit. I personally have code from 1988 that's still running in this application.
Sigh...