You have to say (decode-coding-region (point) (point-max) 'utf-8) first
for that to work.  I.e.,

  (url-retrieve
   "https://dict.youdao.com/suggest?q=accumulate&le=eng&num=80&doctype=json"
   (lambda (_status)
     (goto-char (1+ url-http-end-of-headers))
     (let ((buf (current-buffer))
	   (end (1+ url-http-end-of-headers)))
       (with-temp-buffer
	 (insert-buffer-substring buf end)
	 (goto-char (point-min))
	 (let ((j (json-read)))
	   (erase-buffer)
	   (json-insert j)
	   (write-region (point-min) (point-max) "/tmp/acc2.json"))))))


-- 
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   bloggy blog: http://lars.ingebrigtsen.no