> <span lang="de" xml:lang="den"> 
> 
> to the german part. This should works with screenreaders. Is it 

span is the last resort inline element.  The way you talk about
the german part, it seems that div is more likely to be the right
carrier.  In other contexts, q is likely to be a better carrier than
span (for HTML 4.01).

> possible the add an attribute who defines a document as multi 
> language?

The target language audiences of a document are specified using the
Content-Language HTTP header.  This does permit multiple languages.  
If a language appears in Content-Language, it should be the case that
someone who only understands that language should be able to benefit
from the document.

Any new, multi-lingual, attribute should probably be inserted through
the general meta-information mechanisms.

This feels like a w3c-wai-ig list question, rather than an www-html
question.