This is an indian style number format, where 100,000
(written as 1,00,000) is one "lakh".
Translating and simplifying the Objective-C code from Currency Formatter for different Locale in iOS gives:
let formatter = NSNumberFormatter()
formatter.numberStyle = .DecimalStyle
formatter.locale = NSLocale(localeIdentifier: "en_IN") // English (India) locale
let number = 12345678
let string = formatter.stringFromNumber(number)!
println(string) // 1,23,45,678
Update for Swift 3:
let formatter = NumberFormatter()
formatter.numberStyle = .decimal
formatter.locale = Locale(identifier: "en_IN") // English (India) locale
let number = 12345678
let string = formatter.string(for: number)!
print(string) // 1,23,45,678