Metric system as default game option

[scarf005]

There's no changes for users already playing BN

it's only default value changes.

For users who newly installed BN

• default interface option will be changed to metric system (km/h, celsius, 24h).
• they can be reverted to US customary unit (fahrenheit, lbs, etc) anytime on the interface tab of game settings.
• game's default volume unit already is liters, so it would stay regardless of poll result.

see #2510 for details.
poll will be open from 2023-03-31 23:11 UTC+09:00 to 2023-04-08 12:00 UTC+09:00.

[Coolthulhu]

Alternative: try to get it from current system locale if it isn't set to anything.

[scarf005]

i'm not against that, but we could do #2510 first then merge DDA changes later.