ID for language-invariant strings

[Frank Ellermann]

Peter Constable wrote:

> What are others thoughts?

More unclear tags remotely in the direction of "und", 
"mul", "mis", "i-default", "zxx", and "" (empty) are
not attractive.

How about broadening one of these existing tags for
your purposes ?  This won't work for "", obviously. 

If what you need is an ISO synonym for "i-default"
it makes sense, but it might be also a bad idea for
"old" language tag applications.  

Potential users of an ISO 639 name for "i-default"
likely don't use RFC 4646 language tags, so if you
think this apparent gap has to be closed go for it.

 Frank