I write bi-lingual, two-column articles (English & Chinese), which
must flow through many pages. I want the result to be English always
in left column, and Chinese always in right column, with contents of
both languages approximately  opposite each other.

I can use "column breaks" for English materials to go left. But how
can I input the Chinese materials go on the other column? Perhaps
using Text-box is an option, but is there a more efficient way to do
it?

I think the easiest way is to use text boxes.  Set up each page so
that you have two text boxes on it.  The left one will contain the
English text and the right one will contain the Chinese text.

To get the text to 'flow' from the box above to the one below use the
'link' facility.  That is click on the border of the first box and a
context menu will appear, then click 'Create Text Box Link'.  The
cursor will change to a coffee mug symbol and you just move it to the
box you want to flow information into.  Click in that box and now text
will automatically flow from the first box to the second.  You can
repeat this so that the text flows from the second box into a third
box, and so on.

Setting up pages like this will mean you have two 'columns' on your
pages and the text flows vertically from the one above to the one
below for both the English and the Chinese text (there will be no
problems with the English text trying to flow into the Chinese box as
would happen if you tried doing this with columns)

Two refinements:

1.  Switch off the borders on the text boxes so that the text looks
like two columns rather than a pair of boxes.  Do this by right
clicking on the edge of a text box and in the context menu click on
'Format Text Box...'  click on the first tab 'Colors and Lines' and in
the 'Line' > 'Color' drop down box, change from Black to 'No Line'.  
'OK' out and the edge of the box will have disappeared.

2.  If you are creating multiple documents with this format, it would
be a good idea to create a template document with all the formatting
already done.  Why not create a 10 (?) page document.  Begin by
pressing [Ctrl]+[Enter] ten times to create ten blank pages, create a
pair of text boxes on each page, link them vertically as described
above and then turn off their borders and then save the empty
document.  Whenever you start a new project, open this file and
immediately save it with a new name and then just work on the
translation - all the formating will have been done for you.

just a little idea.  If you where creating 2 documents 1 in english,1
in chinese, with diffent margins so that when you print the document
the english part will be printed in the left side and if you put the
paper back in the printer you could print the secoud document in the
right side.

This is a weakness of Word's column handling. A column break will take
you from col 1 to col 2, but another column break will take you to col
1 *on a new page* -- at least it did when I tried it just now.

One way would be to use Tables, with each paragraph a row in the
table. This would keep the English and Chinese opposite each other,
but I don't know how Word will handle Chinese (vertical) writing, if
this is wanted.

Word's colum style is called 'newspaper' columns. WordPerfect had, as
another option, 'parallel' columns, which is exactly what you want.
Perhaps other word processors have a similar style, which will work
better than Word.