I will update zh-hans after this PR is merged.
Also, should the "No file" in title bar be translated?

Update:
https://github.com/natural-harmonia-gropius/uosc/tree/intl-topbar

I've just pushed support for dynamic localization strings. It's just a cached lua string.format(), so if you rebase, you can swap this to:

device.description:sub(1, 7) == 'Default' and t('Default %s', device.description:sub(8)) or t(device.description)

and

t('%d channels', track['audio-channels'])

Used device.description:sub(9), 9 instead of 8, to prevent a double space, since device.description already has a space in it Default (device name).
8 means having the translation string be Default%s to prevent the double space, looked uglier than Default %s.

Whoops, missed that you're using %d in there. I guess just switch %s that are not %s in your other PR.