The feature exists!
In Tiki 1.9 it's actually possible to define a wikipage as being a translation of another wikipage. Click on translation tab on tiki-view pages.Then, when viewing a page you can switch to a translation, if available, choosing the language in the dropdown menu, next to the page title.
Two feature are still missing though:
- define a language as the default language at a site level and/or page level
- automagically switch to translation corresponding to your preference for wiki GUI, if available. If not diplay default language.