This is pretty bad for SEO, because Google needs the language identifier to index the content correctly. An English user will get search results from Google in their language defined to be English. So if the language variable is set incorrectly by Xenforo then the content will not come up in Google in such search. At least not as ranking.
As xenforo does not allow for the same node to have content in different languages, most websites have setup categories and child nodes dedicated to a language. i.e.
- English category
- - English node 1
- - English node 2
- - English node 3
- German Category
- - German node 1
- - German node 2
- - German node 3
If we would be able to set a language per node then we could give Google the needed language variable.
But there are a lot more benefits, because as soon as a node language is defined addon developers can start building multilingual addon functionality.