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Nbviewer is just a service that lets you host notebooks, there is no computation there.

> Does anyone have experience building (Supervised / Unsupervised) models using something other than Python (including deployment)?

XGBoost/LightGBM have a C API and can be used from pretty much anything, deployment is not a problem. Practically building models is more about dealing with data, the ecosystem tends to revolve around Python and R for that reason.