variants of language codes

Hello,

does anybody know how to specify the language code for the new 
spelling rules in Germany in order to use them in the property 
>language<? Or more generally: The xsl:fo spec refers for the 
language codes to [RFC1766] and if I understand the explanation 
there correctly I can define variants as I like? p.e.: language="de-
newspell" would be ok?

Fotis Jannidis