What is wrong with that url (except default.aspx)? It's widely used and I like if I'm not forced to some region or locale based on my IP and Accept-Language. Well, I still am but I can change it easily if they give me this url (or even better link on a page).
Accept-Language is trivial to change; on Opera, it's one of the three things on the first tab of Preferences, and I can set it for all sites rather than on a per-site basis.
The content is (should be) identical, and the recipient of your link would most likely want to read the content in his/her preferred language.
If you really wanted to point out to them some discrepancy in the content of some particular language, then just tell them "switch your language to Chinese and look at xxx here".
On sites where linking to a particular language version of some resource makes sense (e.g. Wikipedia), by all means, accept a ?lang=de parameter… it's exactly as much mechanism as a directory component, but it doesn't imply any hierarchical structure, and can be dropped to obtain a "canonical" URL.
Microsoft itself provides a prime example for this: Knowledge-Base articles are often poorly translated and you really want to link to the english version if there is no translation available. Implicit content switches are really bad for the user experience.
Also, language versions of big websites often differ hierarchically as well, with the differences in hierarchy being directly bound to the language. On Wikipedia, the problem is so big that it is actually moved to the domain (the highest level of the hierarchy).
(should be): yes. It should be. In the ideal world of people that constructed RFCs. Sadly, there is a real world out there.
In the end, with my users hat on: I don't care. I want to send a link and I assume that my peer sees exactly what I see. I don't want to care about the details of your technical implementation of your website and whether it is structurally sound in the grand scheme of the interwebs. I also don't want to say "and switch to..." every time I send a link.
