Hi @andreasft,
Welcome to the RStudio Community Forum.
Maybe the details in this link may help:
https://www.r-bloggers.com/2013/01/r-and-foreign-characters/.
There are also some other things to try in this post:
https://forum.posit.co/t/issues-with-french-text-showing-up-incorrectly-in-r-script-in-rstudio/59896