Swift: Convert struct to JSON?

I created a struct and want to save it as a JSON-file.

struct Sentence {
    var sentence = ""
    var lang = ""
}

var s = Sentence()
s.sentence = "Hello world"
s.lang = "en"
print(s)

...which results in:

Sentence(sentence: "Hello world", lang: "en")

But how can I convert the struct object to something like:

{
    "sentence": "Hello world",
    "lang": "en"
}

Swift 4 introduces the Codable protocol which provides a very convenient way to encode and decode custom structs.

struct Sentence : Codable {
    let sentence : String
    let lang : String
}

let sentences = [Sentence(sentence: "Hello world", lang: "en"), 
                 Sentence(sentence: "Hallo Welt", lang: "de")]

do {
    let jsonData = try JSONEncoder().encode(sentences)
    let jsonString = String(data: jsonData, encoding: .utf8)!
    print(jsonString) // [{"sentence":"Hello world","lang":"en"},{"sentence":"Hallo Welt","lang":"de"}]
    
    // and decode it back
    let decodedSentences = try JSONDecoder().decode([Sentence].self, from: jsonData)
    print(decodedSentences)
} catch { print(error) }