You need to set the correct Unicode text to your text entity. For example:
char* txt = ""; // Some multi-byte Japanese text in CP_ANSI_932
OdAnsiString mbStr(txt, CP_ANSI_932);
OdString unicodeStr(mbStr);
OdDbTextPtr textEnt; // Some database text entity
textEnt->setTextString(unicodeStr);
While saving, drawing text content will be decoded automatically in a suitable format by ODA.