Provide confguration of default language

[EnricoMi]

Given a default language or a sequence of them would allow sources to pick only a single string value for multi-language predicates that have a @lang directive. Querying for <predicate>@lang1:lang2:. would pick lang1 first, then lang2, then no language, finally any other language. This make the connector pick certain languages when reading from Dgraph. Predicate name should then not include any language tags. This would work with any source and mode.

This should then allow:

spark.read.option("dgraph.language", "en").dgraph.triples(target).show(false)

subject	predicate	objectString	objectType
1	title	Star Wars: Episode IV - A New Hope	string
3	title	Star Wars: Episode V - The Empire Strikes Back	string
6	title	Star Wars: Episode VI - Return of the Jedi	string

spark.read.option("dgraph.language", "zh").dgraph.triples(target).show(false)

subject	predicate	objectString	objectType
1	title	星際大戰四部曲：曙光乍現	string
3	title	星際大戰五部曲：帝國大反擊	string
6	title	星際大戰六部曲：絕地大反攻	string

spark.read.option("dgraph.language", "de").dgraph.triples(target).show(false)

subject	predicate	objectString	objectType
1	title	Krieg der Sterne	string
3	title	Das Imperium schlägt zurück	string
6	title	Die Rückkehr der Jedi-Ritter	string