I'm running the xfce on Manjaro installation I did from an image last month. While I have done some updates, customizations, and additions of software, I think I recall this being an issue from the initial installation. Not sure whether it's a Manjaro or xfce issue, but I am not having it on another box with xfce on Opensuse.
The names for the months and days of the week seem to be in a language I don't recognize and can't find on Google. For example, August is labeled Leqeeni, and the days of the week are Sab, Aca, Etl, Tal, Gam, Arb, Kam, and Gum. The words for today and yesterday are also tranformed into similar puzzles. As far as I can tell, this happens in all apps that display the names of days and months, including both the calendar and the file manager. The actual dates are not affected, as things sort correctly, and if I change settings so that the numbers are shown instead of names they are correct.