Audacity is mostly written in C++
The source code is freely available and there are basic instructions available for build it: http://wiki.audacityteam.org/wiki/Developer_Guide
Audacity is a large and complex project (over 200,000 lines of code), but you will find many examples of audio effects in /src/effects/
Alternatively, Audacity includes a scripting language called “Nyquist” which is based on LISP (XLisp). For a short project you may find that writing an effect in Nyquist is a good option. It is relatively easy to learn and has a wide range of DSP primitives which allow rapid development. See: https://forum.audacityteam.org/t/manuals-and-reference-material/32817/1
Another option for rapid deployment could be Faust: http://faust.grame.fr/