A friend bumped into what appears to be a very irritating problem yesterday with the PHP date_format() function, which is used by format_date() to show date and time strings on Drupal. This function uses the "S" format character, which returns the english ordinal number suffix for the current day of the month. E.g: "st" on the first day, "nd" on the second day, and so on. (And the date() function does too, coincidentally)
The problem is that when you're working in a non-english locale, the ordinal suffixes returned remain the english ones. Oops.