Unicode and accessibility

I would like to ask the list about potential or already existing  problems or challenges for accessibility caused by the use of Unicode.
 
Let us take Google as example. It returns search results in many different languages on the same page, and the result page uses Unicode.
 
At the moment change of natural language is not included in the mark-up. Since the user can choose to get results in a particular language only it would probably be possible for Google to indicate change of natural language automatically even when many languages are used in the same page and the page is generated from many different language sources.
 
It is probably less realistic to expect smaller or ordinary websites and web services to be able to include mark-up for change of natural language when documents are generated on the run from many language sources including interaction with users, like commentary and debate, etc.
 
Now consider a modern word processor like MS Word. Even if 10 different languages are used in 10 paragraphs on the same page, the spell checker has no problem identifying the change of natural language and to apply the right dictionary for each paragraph. No indication of change of natural language is needed by the author.
 
Maybe it is more realistic in many situations to leave indication of change in natural language to user agents than to expect web page authors to do the job. Web page authors should probably still indicate change of natural language in web content made by themselves, but it is probably much more convenient and realistic to leave this task to user agents for many types of generated content. Why not leave the job of indicating change of natural language to a handful of user agents and save millions of web page authors for a lot of work?
 
The above is just one example of problems or challenges for accessibility arising from or made more common by the use of Unicode. I would like to hear of other cases, and if it is more realistic in many situations to leave detection of change in natural language to user agents.
 
Best regards,
Jesper Tverskov
www.smackthemouse.com