how do I get Vietnamese language?

I'm not sure this is the correct forum for this question, but you need to go into System preferences,

International>Input Menu.

Scroll down to Vietnamese, and check the box to it's left.

When you click in the menu bar at the top right, you should have a language selector (mine indicates British with the Union flag). Click on that, and you should be able to reselect from your list of preferred languages, and also select the keyboard viewer for that language.

Almost forgot something.

At the top of the Input Menu window, check 'Keyboard Viewer' and from the bottom of the International page, check 'Allow a different Input source for each document' which will allow you to
change from Australian to Vietnamese (assuming that you have also checked Australian from the list of languages).

This advice is good but the question is how do you input the many tone marks and diacritics in Vietnamese?

It would be tedious to have to click on each additional character in the glyph's pallette.

This advice is good but the question is how do you input the many tone marks and diacritics in Vietnamese?

It would be tedious to have to click on each additional character in the glyph's pallette.

You don't use the character palette, you use the keyboard, after you switch the layout to Vietnamese. If you do this, and open Keyboard Viewer, you will see that that diacritics are added via deadkeys. E.g. if you hit "a", you then select a key from the top row that corresponds to the diacritics you need, like àảãáạ. Or hit ă and then select ằẳẵắặ.

Does this also work for the low diacritics?

By "low" diacritic do you meant the dot below? That is the only one I am aware of, and you should be able to see it in my earlier posting: ạặậ.

Let me know if I have misunderstood.

I thought there was also a hook, but I'll check.

There's no hook underneath, but there is the hooked o and u, which can be handled just like any other vowel, e.g.: ưừửữứự

Plus đ and ₫ which have their own keys.

Excellent Tom.

Does this also work for the low diacritics? If so I must tell my Vietnamese translator who switched to PCs.

I thought there was also a hook, but I'll check.

Thanks Tom, you should be bronzed and put in the Apple Hall of Multi-Cultural Fame!

Yes I checked too.

My faulty recollection, its usually top right, must be Nang's weird typing. He occassionally uses the comma instead.