Creating a interface

What is the best language or tool to create a interface for a program written in R (Programming Language ) ?

What do you mean by interface? As in a frontend of some kind (for web, like Shiny, or desktop app like the TCLTk bindings)? Or as a GUI for working with R? Or as an actual interface/bridge to another language, allowing R code to be called from something else? The word interface has multiple very specific meanings, need to be a bit more specific I’m afraid