Maybe we can refer to SciTE (see http://scintilla.sourceforge.net/SciTEDoc.html about code.page), which also depends on scintilla. In SciTE, I can edit Chinese perfectly through adding
code.page=936
character.set=134
to SciTEGlobal.properties, not using unicode. Of course, we can add code.page=65001 for using unicode. Both work well in Windows.
For GTK+ in Linux, it seems that just using unicode can solve my question. So using unicode in C:B seems to be the best way.