So what's the difference between putting this into an official extension and having this feature in the core?
In both cases you'll have to support that localised rules work with the other extensions. (Which shouldn't be a big problem, because extension developers would have to use the aliases for building proper urls anyway)
This would be used for all forums having a non-english audience. It's just consequent to have it in the core and it makes life easier for admins.
Looking from the perspective of a casual admin, I would ask myself why I can download language packs and install them right away, but for using the localised url schemes packaged with the language files, I need to install an extension first. Makes no sense to me.
http://www.fluxbb.de - Deutschsprachiges FluxBB Supportforum