implement different language tag parsing for JSON

[pkiraly]

There are at least two types of language tagging Europeana uses:

implicit

"dcCreator": {
  "nl": [
    "Stichting Xeno-canto voor Natuurgeluiden"
  ]
}

explicit

"dcCreator": [
  {
    "@lang": "nl",
    "#value": "Stichting Xeno-canto voor Natuurgeluiden"
  }
]

We should provide a value how to interpret the data: – asLanguageTagged: implicit – asLanguageTagged: explicit