WMG / 2009: Lost Memories

Sakamoto never restored the timeline. He merely created a new timeline merely by his appearance.
The fact that the new timeline had the North and South Korea reunite in 2008 may imply that Sakamoto even tampered on the communist faction, eliminating those who he think will do more harm than good in a liberated Korea.
China and Japan substituted places in World War II.
It would make sense since the Chinese at that time were supplied by Nazi Germany.
Sakamoto's mother secretly taught his son Korean.
That would explain how he is able to speak Korean fluently.