Since December 2014, the bank's headquarters have been [located] in Paris.
The bank's headquarters are in Paris and have been since December 2014.
You will notice that I have used "headquarters" and followed it with the plural. There is a BrE vs AmE here. I understand from one of our AmE members that it's "headquarter" + singular in AmE.
In BrE, even if it's just one building, we still use "headquarters" + plural.
Student or Learner