This is an offspring of r-lib/devtools#2377. Consider this example:

env -i LC_COLLATE=  R -q -e 'withr::with_collate("en_US", sort(c("_a", "a")))' # normal run
env -i LC_COLLATE=C R -q -e 'withr::with_collate("en_US", sort(c("_a", "a")))' # as run by R CMD check

On Linux, these two output different results:

> withr::with_collate("en_US", sort(c("_a", "a")))
[1] "_a" "a" 
> withr::with_collate("en_US", sort(c("_a", "a")))
[1] "a"  "_a"

Since the documentation of with_collate does not say anything about the need to take existing environment variables into account, I wonder if Sys.setenv should be added to ensure reproducibility regardless of environment variables.