Sys.setlocale("LC_TIME", "cy_GB.utf8")
lubridate::ymd("2021-03-19") %>% strftime("%d %B %Y") %>% lubridate::parse_date_time("%d %B %Y")
# [1] NA
# Warning message:
# All formats failed to parse. No formats found.
# Same result if you try lubridate::fast_strptime("%d %B %Y") or lubridate::dmy()
For context, "cy" is the ISO 639-1 language code for Cymraeg/Welsh.
So far so good, but...
and more worryingly...
This is using the Ubuntu Welsh language pack: