It's possible to get Vietnamese language in that dropdown. It's kind of pointless to do so unless you are also installing Vietnamese Hunspell dictionaries. You don't say which version of ID you are using, but if you are in CS5 or above, I think, you can download and install Vietnamese dictionaries. I don't know why you would need to do so - Vietnamese average word length is so low that you wouldn't need hyphenation, and while it might be nice to flag potential misspellings for your translation provider, they should have much better dictionaries than you, and also they should know how to use them. Should.Also, spelling in Vietnamese is kind of a minefield - due to radical changes in Vietnamese language circa the mid-70s, so addressing aging North American Vietnamese populations is radically different from addressing Vietnamese populations in Vietnam. Your selection of languages implies that you are working on translations for use in North America, not in-country. So I'd suggest that you mark the Vietnamese as [No Language], turn off hyphenation, and move on with your project. Some of the Adobe Pro fonts that came with the Creative Suite support Vietnamese, but not all of 'em. If you were on Windows, I'd suggest a handy tool called BabelMap that will let you figure out which of the fonts you already have installed will support Vietnamese. I think you can use Character Map to in Mac OS to do the same thing.
You'll most likely need hyphenation in Russian, but most hyphenation engines get it wrong. If you can get Hunspell Russian hyphenation dictionaries installed, do it. Proximity hyphenation of Russian is abysmal. Once again, many Adobe Pro fonts will have Cyrillic support.
Lastly: It feels unacceptably harsh for me to say it this way, so my apologies in advance: don't even bother tackling the Khmer yourself. Job it out to someone who knows the language - perhaps the firm providing the translation? There are all kinds of pitfalls in Cambodian typesetting. For example, there aren't spaces between words in Khmer. If you get a translator who actually keys zero-width spaces, you've hit the jackpot. But there is no guarantee that you will get a translation team that is even using a Unicode font. The world of pre-Unicode custom-encoding hacked-fonts Khmer typography is a mess. So I'd say that you are best off leaving that particular job to a pro.
Thanks for the great and detailed advice! I'm using CS6. So dumb question: where do I get the Hunspell Russian and Vietnamese hyphenation dictionary? Still don't see Vietnamese in dropdown -- list stops at Ukranian....? And yeah, looking at their sample of Khmer, yikes! I'll definitely leave that to them to layout.
No problem! I've spent decades (okay, almost exactly two) accumulating little scraps of multilingual DTP knowledge, it feels good to info-dump on someone who actually wants to hear about it.
Anyhow: I said that it was possible to get additional languages into the list, and also possible to install Hunspell dictionaries, but I neglected to say exactly how.
Here's exactly how:
I typeset a lot of Khmer, but if my immediate manager wasn't herself Cambodian it would be markedly more difficult.
Thanks a lot Joel! This is going to be an interesting project to say the least.