It isn't regional; it's linguistic and ethnic. Typically, translated from Hebrew or Yiddish it will be
Yakov (you've got
Yaakov on this site).
Jakob is derived from this, then we come to
Jacob. These are phonetic transliterations. Perhaps you see more people called/spelled
Jakob in certain regions where Hebrew or Yiddish is spoken more often than English, but that doesn't make it a regional variant. If you were to look at Orthodox Jews in
Brooklyn, NY, you may encounter more Jakobs or Yakovs than you would Jacobs. These are ethnic and linguistic distinctions -
Brooklyn is an *English*-speaking borough.