How to join objects within array of json files by matching key:value?

Given file1.json and file2.json with objects sharing a common "lang" properties with identical values :

[
  { "lang": "Inari Sami", "audios": 78 },
  { "lang": "Tofalar", "audios": 42 },
  { "lang": "Kumyk", "audios": 305 }
]

[
    { "lang": "Inari Sami", "isoCode": "smn", "genderFemales": 1, "genderMales": 5, "genderOthers": 3 },
    { "lang": "Tofalar",  "genderFemales": 2, "genderMales": 3, "genderOthers": 1 },
    { "lang": "Kumyk", "isoCode": "kum", "genderFemales": 5, "genderMales": 4, "genderOthers": 2 },
    { "lang": "French"}
]

How can I get a consolidation (fusion) of those such as :

[
{ "lang": "Inari Sami", "audios": 78, "isoCode": "smn", "genderFemales": 1, "genderMales": 7, "genderOthers": 1 },
  { "lang": "Tofalar", "audios": 42, "isoCode": "xal", "genderFemales": 2, "genderMales": 6, "genderOthers": 0 },
  { "lang": "Kumyk", "audios": 305, "isoCode": "kum", "genderFemales": 5, "genderMales": 7, "genderOthers": 1 },
  { "lang": "French"}
]

Here is one way:

jq -s 'add | group_by(.lang) | map(add)' file1.json file2.json

^{Online demo}

and another:

jq -n 'reduce inputs[] as $obj ({};
  .[$obj.lang] += $obj
) | map(.)' file1.json file2.json

^{Online demo}