As a non-native English speaker I think it will help to expand the current documentation about having several languages supported by the same package. This way we can ease the adoption of a package without restricting (much) ourselfs to a language.
I think it has something to do about the /po folder of a package but I couldn’t understand how to use it/fill it. It is only briefly commented on advanced R (http://r-pkgs.had.co.nz/misc.html) and on the R manual itself (https://cran.rstudio.com/doc/manuals/r-devel/R-exts.html#Internationalization 1.8.3 Preparing translations)