Taxonomy is normally pretty simple by default except that it's also called vocabulary (but never terminology although the tags are called terms) and that it can be attached to nodes as fields (using Content Taxonomy Fields to make CCK fields which gives you a bit more control) or not, or both.

Adding language makes it complicated though. Drupal can be said to support languages other than English, multiple-language sites, internationalization and translation. But it's not always user friendly or clear even to developers. Such is the case with taxonomies.

Let's say you have a site that's in two languages, English and Vietnamese. You have translated the interface of the site as well as nodes so that URLs are consistent. To switch language you just add the language code to the front of the URL. So you have a taxonomy with terms in your site's primary or default language. But you want to use the same terms by ID rather than a different set of terms.

1) Edit the vocabulary. Set "Localize". This doesn't set a language to any terms or anything, they are just assumed to be in the default language already.

[Look in term_data and confirm the language column is still empty.]

2) Refresh strings in Translate Interface. Now if you search taxonomy for a term it should appear.

[Look in locales_source for location = "term:$termid:name" where $termid is the tid of a term in your vocabulary. This means it's ready to be translated. After translating, it should be in localtes_target with the same lid. But the translation column is a blob so you won't be able to see the translation directly depending on your mysql client.]

3) I recommend installing the Translation Table module instead of searching for each new term in the regular translate interface. Translation table shows up as a new tab in the translate interface and you can select a vocabulary to see all of its terms in one place.

4) If you use terms in arguments in URLs you'll need something further. Otherwise, all URLs will use the term in the site's default language.

5) One last thing: Don't use t() to translate a term. Use i18ntaxonomy_translate_term_name on a term object (like if you get a vocabulary object from taxonomy_get_tree).

Writing Vietnamese is a pain. You have several different input systems to choose from (VNI, Telex, VIQR, etc.) and they all require some effort to memorize and get used to. But as it turns out most Vietnamese text can be understood without those accent (diacritic) marks at all. Vietnamese people are accustomed to texting (sms) each other using Vietnamese words written without marks - because their phones often don't support Vietnamese characters. But it's better to have the marks and emails and posts on forums often do. GMail even supports a Vietnamese software keyboard built into the interface now.

But if Vietnamese people can understand Vietnamese without diacritics, can computers? Turns out there is software that can take unaccented Vietnamese text and ADD the diacritics!

Let's take some text:

Chuyến phiêu lưu khám phá bỏ nhà ra đi đầu tiên của bạn vào năm bao nhiêu tuổi? 
- Nghĩ lại thì thấy hồi xưa mỗi lần mà bị mẹ la là hay giận, bỏ nhà đi lắm, vì lúc nào cũng nghĩ mình đúng hết. Giận thì giận nhưng mà đi lang thang rồi lại về, hoặc về trong tình trạng được tìm thấy và lại tiếp tục bị mắng :D

Then we strip the accents and put it into a few websites to see the results.

http://vietnameseaccent.com/

Chuyến phiêu lưu khám phá bỏ nhà ra đi đầu tiên của bạn vào năm bao nhiêu tuổi?
- Nghĩ lại thì thấy hồi xưa mỗi lần mà bị mẹ là lạ hay gián, bỏ nhà đi làm, vì lúc nào cũng nghĩ mình dùng hết. Gian thi giản nhưng mà đi lang thang rồi lại về, hoặc vê tròn

http://vietlabs.com/vietizer.html

chuyến phiêu lưu khám phá bỏ nhà ra đi đầu tiên của bạn vào năm bao nhiêu tuổi?
- nghĩ lại thì thấy hồi xưa mỗi lần mà bị mẹ là là hay gian, bộ nhà đi làm, vì lúc nào cũng nghĩ mình đứng hết.
gian thì giản nhưng mà đi lang thang rồi lại về, hoặc về trọn

http://www.easyvn.com/tiengviet/index.php

Chuyến phiêu lưu khám phá bỏ nhà ra đi đầu tiên của bạn vào năm bao nhiêu tuổi?
- Nghĩ lại thì thấy hồi xưa mỗi lần mà bị mẹ là là hay giận, bỏ nhà đi lắm, vì lúc nào cũng nghĩ mình đúng het. Gian thì giận nhưng mà đi lang thang rồi lại về, hoặc về tron

The results are nearly the same except for the last word: 'tron'. This is because the real word is "trong" but it got cut off in the de-accenting process! So each different software took a different guess as to what the word was, but it was the wrong word to begin with.

All in all, they do a pretty good job and probably better than even some native Vietnamese speakers due to the fact that some tones are mixed up!