Last week on a different installation of i18n and Drupal 5.0, I followed the instructions here http://drupal.org/node/83514 to get the Primary Links to switch languages when the visitor switches the language of the website. It seems to have worked.
(At least, I think it worked. According to my own message at that thread, I claim it didn't work for me. But that is because I think I was looking at the menu from an Administer screen -- and not from a regular page. I have come to realize that you must always view the navigation blocks from a regular page. Otherwise, you will (always?) see *both* languages in the Primary Links navigation bar.)
However, through trial and error I think I have discovered another way to accomplish the same thing -- but without adding stuff to /sites/default/settings.php. If someone has already posted these, I'm sorry. I didn't see detailed instructions like this. Also, I'm posting them in case I've done something horribly wrong that will have unforseen (to me) effects on my Drupal website.
Here are my instructions for how to create bilingual Primary Links (in English and French):
1. From any English page, click Create content
2. Click Page (or Story, whatever)
3. In Title box, enter About
4. In Multilingual settings, make sure English is selected
5. Enter Body text (optional for the purposes of this demo)