Right way to write string into UTF-8 file?

[snippsat]

Should I use another method than "dump"?

Not sure because don't know where i goes wrong,don'not have file you use.
You should not use f in both read and dump.

Here a example where i use a geojson file and put in François.

import json
from pprint import pprint

with open('map.geojson' , encoding='utf-8') as f:
    data = json.load(f)
    pprint(data)

Output:
{'features': [{'geometry': {'coordinates': [[8.9208984375, 61.05828537037916],
                                            [9.84375, 61.4597705702975],
                                            [10.7666015625,
                                             60.930432202923335]],
                            'type': 'François'},
               'properties': {},
               'type': 'Feature'}],
 'type': 'FeatureCollection'}

Dump data.

import json

with open("data_file.json", "w", encoding='utf-8') as f_out:
    json.dump(data, f_out, ensure_ascii=False

Content of data_file.json.

Output:
{
  "type": "FeatureCollection",
  "features": [
    {
      "type": "Feature",
      "properties": {},
      "geometry": {
        "type": "François",
        "coordinates": [
          [
            8.9208984375,
            61.05828537037916
          ],
          [
            9.84375,
            61.4597705702975
          ],
          [
            10.7666015625,
            60.930432202923335
          ]
        ]
      }
    }
  ]
}