This is something about Drupal i81n/localization I found the answer to and need to write down somewhere, in case it helps someone else or I forget it.
If you're trying to allow users to select different languages in their user settings, you've got more than one language installed, are using Drupal 6 and it isn't working. Go into Administer -> Site configuration -> Languages and change Language negotiation to Path prefix with language fallback.
If you had the Language negotiation set to None, because -- like me -- you thought that would allow users to change their language preferences, you were wrong. The problem is this help text on the Languages page is completely misleading:
None. The default language is used for site presentation, though users may (optionally) select a preferred language on the My Account page. (User language preferences will be used for site e-mails, if available.)
It should read:
None. The default language is used for site presentation, though users may (optionally) select a preferred language — for site e-mails, if available — on the My Account page.
Apparently this will be fixed in Drupal 7. A bug to change the wording in six was rejected as a duplicate. 
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ppritchard posted this at 15:16 — 9th July 2009.
They have: 1 posts
Joined: Jul 2009
A note to others finding this via Google (thanks JB!):
I had to ensure that the URL setting for each of the translated versions of my node was the same as my original node (in my case, "home"). I was being shown a "page not found" error when it was unset and defaulted to "node/123".
JeevesBond posted this at 17:14 — 11th July 2009.
He has: 3,895 posts
Joined: Jun 2002
Heh, Google found this already? w00t! Welcome to TWF by the way, and thanks for adding what you found out.