In Drupal 5 the language prefix was always visible (when using the i18n modules), while as in Drupal 6 the built in i18n support provides no prefix for English pages and prefixes for all other languages (when prefixes are enabled). It's great that Drupal 6 doesn't force you to use a language prefix for all languages, unfortunately it'd make more sense if the language without a prefix would be the site's default language, and not always English. It'd also be good (especially for backwards compatibility) to offer an option to always use language prefixes.
Already long ago someone suggested that the default language would be the one not using a prefix http://drupal.org/node/146084#comment-558996 , still this wasn't changed until the final release, thus it could be temporarily fixed through the i18n modules until Drupal 7 is released (hopefully fixing this once and for all).
I also noticed that the usage of prefixes easily causes problems with URL-aliases, apparently also other similar issues appear (according to issue #146084 linked above).
Comments
Comment #1
jose reyero commentedYou can set prefixes for every language (or none) on the language settings page + I don't understand what you mean is missing. The language settins in Drupal 6 core are flexible enough.
Anyway, this was a design decision for Drupal core and we better won't mess that in contrib modules, if you want to change that, file an issue for Drupal core.
Comment #2
nedjoSee these issues on Drupal core: #338055: Ensure non-default languages always have a path prefix and #146084: Default path prefix for English (and DBTNG it).