Due to its popularity, the Harry Potter series of books has been translated into dozens of different languages from around the world. Given the books’ setting in Britain, heavy use of wordplay & allusions, and all of the invented words, translating the books accurately and faithfully was difficult.
Translators weren’t given a head-start — they had to wait until the English editions came out to begin the difficult and lengthy task of adapting the books. Working day and night, translators were racing against intense deadlines. Harry Potter and the Order of the Phoenix, the longest book in the series at 870 pages for the US edition, was originally published on June 21, 2003. Its first official translation appeared in Vietnamese on July 21, 2003. Not long after, the Serbian edition was released in early September 2003.
Also not mentioned in the video is all of the foreshadowing Rowling uses in the first few books in the series that pays off in later books. Since the translators probably didn’t know the plot details of the later books, some of that foreshadowing might have been edited out, downplayed, or misinterpreted.