Which steps are needed for an Unicode program in C++

Which steps are needed for an Unicode program in C++

My problem is I want to write a C++ program which can support typing
Unicode characters in other text editors like LibreOffice, MS Office,
Notepad, ... (because I'm a Vietnamese and my mother tongue language
includes Unicode characters such as: ð, â, à ế, ẹ, ẻ,
...). That means when I use a text editor like those above or any
applications which can support text editing such as Browsers (in address
bar or search bar), Chat applications like Yahoo or Skype, ... and when I
type a key or a group of keys in keyboard, my C++ program will notice that
and convert it in to Unicode character and send it back to text editor.
For example, when I type double 'e' key in text editor, C++ program with
notice that and make it as 'ê' in text editor. My knowledge and idea about
this problem is zero. Please please tell me steps needed or mechanism to
do a such application. I don't know where to start.