Changing decimal and group mark in rmarkdown render - r-markdown

Question

I am working in RStudio (on a Mac) to write reports which are rendered as PDF.
I am working in an English locale.
The report, however, is in German and I would like to use German decimal and group (thousands) marks in the report without changing the locale of the OS (or R).
I would like to do this by setting one global option at the beginning of the Rmd file.
Is there a way to accomplish this?
Up until now I tried
command line in Terminal
defaults write org.R-project.R force.LANG de_AT.UTF-8
This only changed the UI language used by R itself, no by RStudio.
Sys.setlocale("LC_NUMERIC","de_DE")
in an Rmd document produces
Error: package or namespace load failed for ’sf': .onLoad failed in loadNamespace() for ’units’, details: call:
udunits_initCfile.path(.get_ud_xml_dir(), "udunits2.xml")) error: Error in function udunits.init: Can't open
installed, default, unit database Execution halted
Putting
NUMERIC="de_DE"
LC_NUMERIC="de_DE"
in .Renviron does not help either

Answer 1

Quoting ?locales
Setting "LC_NUMERIC" to any value other than "C" may cause R to function anomalously, so gives a warning. Input conversions in R itself are unaffected, but the reading and writing of ASCII save files will be, as may packages which do their own input/output.
Setting it temporarily on a Unix-alike to produce graphical or text output may work well enough, but options(OutDec) is often preferable.
So let's try that:
---
output: pdf_document
lang: de-DE
---
```{r echo=FALSE}
options(OutDec = ",")
library(sf)
knitr::kable(head(mtcars))
```
Dies ist ein nichtssagender Blindtext inklusive R output `r pi` der genau so
lang ist, dass es zu Zeilenumbrüchen mit Trennung kommt.
Result:
Notes:
There is no group-separator, since R does not use that in its output.
I have added lang: de-DE to enable German hyphenation patterns.
Loading the sf package works without problems.