The xref.el library has many variables and functions with `backend' in
them, indicating that they represent language-dependent entities.  Before
the xref API is set in stone, I would consider replacing this term with
'language' as it reads better in the names, it refers to a concept
intrinsic to the cross-references themselves which are language-specific,
whereas backend is just an implementation detail, and it will make more
sense to users of the library who may not be familiar with the concept of a
`backend'.

Just a thought but I know these names will live for a long time.