"locales": {
    "de": {
        "default": {
            "vegetables": [
                "apple",
                "melon",
                "grape",
                "pear"
            ]
        }
    },
    "fr": {
        "default": {
            "vegetables": [
                "apple",
                "melon",
                "grape",
                "pear"
            ]
        }
    },
    "nl": {
        "default": {
            "vegetables": [
                "apple",
                "melon",
                "grape",
                "pear"
            ]
        }
    },
(...)
}

def updateJsonFile():
    jsonFile = open('vegetables.json', 'r')
    data = json.load(jsonFile)
    jsonFile.close()

for item in data["locales"]:
    if item == ["de"]:
        item["default"]["vegetables"] = _MY_CUSTOM_ORDER_DE
    elif item == ["nl"]:
        item["default"]["vegetables"] = _MY_CUSTOM_ORDER_NL
    else:
        ## do nothing ##

    with open('sortedvegetables.json', 'w') as outfile:
    json.dump(data, outfile, indent=4, sort_keys=True)