Hey Björn, just some smaller additions to my previous email response to you:
You probably checked out trunk, not branches/2.3, right? The deeper Translatable fixes unfortunately won't make it into the initial 2.3 release because they need more testing and RC-cycles than the rest of the system.

For the same URLs in multiple languages, the code in question is in Translatable::onBeforeWrite():
http://open.silverstripe.com/browser/modules/sapphire/trunk/core/model/Translatable.php#L483

It would be fairly easy to allow for duplicate URLs across languages, but you'd lose the ability to cache on an HTTP level as the pages would represent more than one "resource" (in RESTful speak).
Example with duplicate URLs: http://mysite.com/group/?lang=en, http://mysite.com/group/?lang=de
Example with unique URLs: http://mysite.com/group, http://mysite.com/gruppe

Duplicate URLs would also potentially harm your Google ratings, as you're providing different content to different clients - any experiences on this?

You suggested in your email response to have subfolders for URLs (http://mysite.com/en/group, http://mysite.com/de/group). We're open for patches in this area, but currently its not implemented.

Generally we could make this enforcing of unique URLs configurable, but currently this implies that we have to append the "lang" GET variable to all URLs in menu generation, tinymce links etc - feel free to play around with this and let me know how you go on.

I'd like to hear some more opinions on this matter, so we can decide if its worth the effort, and consequently open an enhancement ticket request.

BTW I raid a question about this somewhere else I cant remember though, I cant save any of the strings I translate, it gives me an error, so will anyone look into that,

I am translating the CMS to Sinhala

Thanks for your detailed description, Ingo!
I'll try to play around with this!
Suggestions are welcome