Running a bilingual Drupal site

I am setting up a Drupal-based website for a non-profit group that has 
expressed interest in having the site be bilingual (English & Spanish), 
and I must confess I don't know much about how/if Drupal supports this.

A few questions:

(1) Are there translations of the Drupal's built-in interface elements 
to Spanish?

(2) Assuming we have translated versions of the articles on our site (at 
least some of them), how do we establish a relationship between the 
English and Spanish versions of the same article?

(3) How can we allow individual users to express a language preference?

I do this purely as a volunteer effort, so I don't have scads of time to 
spend on it, but if needed I can code up some things. Suggestions on how 
  to accomplish anything that's not already built into Drupal would be 
appreciated.

-Eric


P.S. For the curious, the site is http://www.mn-leon.org. (It's not yet 
running on Drupal, but will be probably in January.)
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