I can confidently say that, on Chinese music enthusiast forums, most people prefer the sound of these 3 languages system: Japanese, German and traditional Chinese.The Japanese region should likely be the initial sound tuning of the Walkman done by the team of Japanese engineers over at Sony, then they pass them on to the domestic Japanese audiophiles for demoing and it is likely these audiophiles really liked the sound.
However when Sony sends these walkman to these overseas chinese audiophiles for demo, they probably find the sound not to their liking, thus Sony has to tweak the sound to suit these chinese audiophiles, but at the same time, if they just tweak the sound to suit the Chinese Audiophiles without any kind of selection, then what about initial preferences of the Japanese Audiophiles?
Also, our forums have Chinese Sony employees, some professional music equipment agents, and even some advanced audiophile users. I've never seen anyone say they received demo firmware from Sony Japan and collected reports, or did surveys or anything like that.
So, I think it's really mysterious, but I think it's an issue with the Android system. Maybe the CPU or memory performs differently under different system languages, causing very subtle differences in power supply. Sound is a mysterious thing – any tiny change, and the DAP sounds different when amplified. Sony probably just tunes their devices for their native Japanese system. When tuning, you always have your own standards, so they wouldn't create so many standards based on different languages. I don't think they have time for that. They probably don't even know there's a difference in sound between different system languages, let alone a huge one. It's really quite interesting.