gh-131146: Fall back to `month_name` if `standalone_month_name`s aren't distinct...

gh-131146: Fall back to `month_name` if `standalone_month_name`s aren't distinct (GH-137674)

Some systems reportedly don't expand '%OB' and '%Ob'.
In this case (and similar theoretically possible ones, like expanding to empty
string or 'OB'), fall back to the month_name & month_abbr.