Last updated November 26, 2010.
Here are basic guidelines which can be good starting points in translating Drupal to your own language. The advice from "Linux for Schools: Localization and Translation" is a good point to consider:
Your task is ... to convey the content, so do not be afraid to localize the original text with something completely different (something that fits, of course). Read the original text, understand its content, and consider how you would formulate the text in your language (without first having heard of the original text).
It is easier to understand Drupal if translators manage to be consistent with the use of words. If you translate a word with different synonyms throughout Drupal, it is difficult for people to understand that you are talking about the same functionality. To achieve this consistency throughout a whole Drupal site, a wordlist for words that occur in Drupal's strings is recommended. The English-Norwegian wordlist for Drupal provides a good example of how this is done. If you create your own wordlist, then the whole translator community of your language can discuss the standardization of Drupal words and expressions into your language. It will be helpful to not set up a new word list, but re-use existing ones from an existing translation project.
It is also important to be aware of the writing style of the localization files. If your translation project has started a conservative and formal style, stick to it. If the style is more colloquial, then follow that style. Feel free to also rewrite existing translations if you think you can formulate them better, but try to stay to the same writing style for consistency.
You also have to keep in mind how much space you have in Drupal for a given text. Sometimes there is enough room that you can get rid of acronyms. Other times, the space is not enough that you need to be a little more creative.
Translate internet terms and technologies and refrain from using the English equivalents. If you are not sure of the equivalent word in your language, try contacting the language departments of respectable universities in your country or a state bureau who takes care of your language. Do not be afraid to use or introduce new terms as long as they are standardized. Remember, when the word "mouse" was introduced (for that hand-controlled device for interacting with a computer), most people thought it was ridiculous.
Other areas which need guidelines will differ from language to language. Please get in touch with the maintainers of your language's translation team or contact your community specific to your language.