The problem probably is that the lua function os.date() returns the value always encoded with your current code page. So, if you concatenate that with the utf-8 encoded string there are two different encodings in the same strings and Rainlendar doesn't know how to decode it (ain't text encoding fun .

I tried to change my locale settings to Traditional Chinese but I can't make the "%a" to return anything else but English and Finnish for some reason. But you can try to solve the problem like this:

sTime = "Chinese characters go here" .. Rainlendar_ConvertString(os.date("%x %a"), "big5")

The "big5" should be the code page for your current locale. If the "big5" doesn't work you can also try "950" as the code page as that's what Windows calls it.

Thank you Rainy, this problem is solved on my pc.
But, if i want the .lua file can applicable to all languages OS,
How can i do?

This is my current method:
In lua file only write s = "Chinese language strings"
And add <time format="%x %a" ...... /> in my_skin.xml

Rainlendar will try to convert the string with the user's current codepage if the utf-8 conversion fails so something like this should change the encoding from local to utf-8:

sTime = os.date("%x %a")
Rainlendar_SetItemValue("LunarCal", "LunarCal.info", "text", sTime)
sTime = Rainlendar_GetItemValue("LunarCal", "LunarCal.info", "text")
Rainlendar_SetItemValue("LunarCal", "LunarCal.info", "text", "Chinese text" .. sTime)

It's working correctly.
Thanks!