It's been a long time since I played with v1. But...
Think about upgrading the v2. Language files stay the same, but v2 is way, way more stable.
You have a couple options for headerFormat -
a. you can either mess with it in the language file (which I assume you are loading, judging by the fact that english is the default).
b. Override options..headerFormat.
c. It looks like just setting options.headerFormat is supposed to override it too... ymmv. Like I said, it's been a while.
The bad news: It is possible that the calendar mode of v1 does not in fact use headerFormat - if that is the case, look for this line, around about 1421, which looks suspiciously hard-coded to me. Which means you'll need to edit the sources directly. Sorry about that.
self.controlsInput.empty().html(o.lang[o.useLang].monthsOfYear[self.theDate.getMonth()] + " " + self.theDate.getFullYear());
Finally, if you do decide to upgrade to v2, it looks like it is also hardcoded - line 160. I'll work on making this an option instead. Edit: the option you are looking for in v2 is overrideCalHeaderFormat
/ calHeaderFormat
.