User blog comment:Xilinoc/Ch. 567 (Rukia be like "I ain't even scared though")/@comment-4957813-20140205215815/@comment-11051098-20140207153235

What can be found on the web:

"Fahrenheit is used in the Bahamas, Belize, the Cayman Islands, Palau, and the United States and associated territories (Puerto Rico, Guam and the U.S. Virgin Islands)"

Seeing as USA and a handful of other countries/island states are the only ones in the world still using the Fahrenheit scale, I would say it's logical for Kubo to use Celsius.