Next, open Notes, and start typing a note. Click the globe icon next to the Space Bar to switch to Japanese, and type a character. When you do, the Notes font will change from Marker Felt to Helvetica. Here's a demonstration video.
[robg adds: This works, and the note will remain in Helvetica even after closing and reopening Notes. You can even delete the Japanese character you typed -- once the font has changed, it won't change back.]

