That’s not possible. In the lang part, Dgraph follows closely to the W3C standard. And I don’t think we would change this behavior soon.
This format above doesn’t exist in the RDF standard as far as I know.
Also, this last example is really out of RDF syntax scope. It has to be like:
_:luke <name> "Luke Friendless"@en .
_:luke <name> "Łukasz Samotny"@pl .
BTW, our parser works with very simple n-triples format. Things like turtle RDF and other standards won’t work.