If the processing goes into servers, it should be robust. Too many applications are buggy. I doubt also. I just agreed that my preferred method wouldn't work well when mirroring is involved. It is still my preffered method, though.
People tend to forget little things, like putting language tags into documents.
If the server can format Content-Language header based on the directory, I can minimise the human factor. I have nothing against putting language tags into the document, but I doubt that many people would do that.