d = {
  "allowed_langs": [{
      "langs": ["java", "golang", "python"]
    },
    {
      "langs": ["java", "golang", "csharp"]
    },
  ]
}

languages = ["java", "golang", "python", "csharp"]

allowed_languages = set()

for allowed_langss in d["allowed_langs"]:
    for each in allowed_langss["langs"]:
        allowed_languages.update(each)

print(allowed_languages)

{'t', 'j', 'g', 'h', 'r', 'o', 'y', 'c', 'v', 'n', 'p', 'l', 'a', 's'}

print([lang for lang in d['allowed_langs']])

[{'langs': ['java', 'golang', 'python']}, {'langs': ['java', 'golang', 'csharp']}]

languages = ["java", "golang", "python", "csharp"]

d = {'allowed_langs': [{'langs': ['java', 'golang', 'python']}, {'langs': ['java', 'golang', 'csharp']}]}
new_d = list({i for b in d['allowed_langs'] for i in b['langs']})

['java', 'python', 'golang', 'csharp']