Turtle RDF isn’t supported at all. It has several syntaxes that the Dgraph’s parser won’t understand.
What you can do is sanitize the data and use the flag --store_xids (that’s only supported with bulkloader).
After store_xids you can use upsert block with this approach https://dgraph.io/docs/mutations/#external-ids-and-upsert-block
Another way around to sanitize your data is by using a tool called “OpenRefine”. And use an option to export it as “custom” (don’t quite remember the name, but it will exports in a JSON format. Which is supported by Dgraph)