Strange locale problems in R after update to Mojave

[mara]

Does this happen both in Vanilla R and in RStudio, or only in RStudio?
(see FAQ below for disambiguation)

Differentiating R from RStudio

Aside: goals for this thread

I'm hoping this will help people disambiguate R from RStudio, and also help with getting questions in the right category. This is a bit of a mashup of the times I've tried to explain this in individual threads, so please feel free to add to this topic for further clarification/point out corrections, etc.

Before putting a code-related question in the #rstudio-ide category or using the rstudio tag, it's helpful to ask yourself: Does this work in the regular R GUI, but not when you're in RStudio? If you're using RStudio, you're using R — RStudio is just an IDE (Integrated Development Environment) in which you are using the R language. (On of my favourite fi…