lack of translations for metadata field names in API responses

[mw0000]

Why are some Dublin Core field names translated in API responses while others are not, even though they have their translations in .po files?

{
    "id": 55,
    "url": "XXX",
    "order": 16,
    "name": "Miejsce urodzenia",
    "description": "",
    "comment": "",
    "element_set": {
        "id": 3,
        "url": "XXX",
        "resource": "element_sets"
    },
    "extended_resources": []
}

{
    "id": 37,
    "url": "XXX",
    "order": 100,
    "name": "Contributor",
    "description": "An entity responsible for making contributions to the resource",
    "comment": "",
    "element_set": {
        "id": 1,
        "url": "XXX",
        "resource": "element_sets"
    },
    "extended_resources": []
}

Element 37 has been translated: https://github.com/omeka/Omeka/blob/cd661c35fcbe1563eb880927635afa972c5a07fd/application/languages/pl.po#L94