I found many programs' source codes in KDE.
All of them do not have the locale codes explicitily.
When kapp begin running, it will get locale and find a mo file in /usr/share/locale/ according to the locale.

the documentation just tells me how to extract a po file from source code.
It doesn't say how to code in KDE program's main function.

And, in KDevelop, there is a menu item "Add Translation...", you can choose a kind of language to generate a po file in po directory, but the file is NULL.
I don't understand it.