My use case is to convert the name of a day to a day of the week number — now testing in English, but ultimately locale insensitive, so relying on to_date() to recognize the day in whatever the database locale is.
Patches are welcome but I'd suggest that you not attempt to incorporate this behavior into to_date. That function returns a date and what you want is a normalized integer (or, in reverse, a locale-specific string).
Both locale-specific so stable, not immutable:
day_of_week(text) : integer
day_of_week(integer) : text
Given that "select to_char(now(), 'Day')" works there should be few, if any, technical barriers to overcome.
You'd need to decide whether to support only the "ISO 8601" numbering scheme (1-7) or to add additional arguments and/or function to number Sunday as 0 instead of 7.