How to implement an enum with string values in TypeScript

I currently work an a single-page application (SPA), where we use TypeScript to access web services. In some places, the sent and received data structures use enums to describe the range of allowed values. I tried to use the enums from TypeScript to access the web services in a strongly typed way:

export interface PersonDto {
    language: Language; 
}

export enum Language {
    English, 
    German, 
    French, 
    Itialian    
}

When serializing a PersonDto instance to JSON, the language field is converted to a number and not a string as required by the web service API:

var person = <PersonDto> { language: Language.German }
var json = JSON.stringify(person);

The json variable now contains:

"{ language: 1 }"

The solution to this problem is to assign the expected string value to each enum member. However, TypeScript only allows numbers as enum member values. But there is a simple solution: Just cast the string literal to any before assigning:

export enum Language {
    English = <any>"English",
    German = <any>"German",
    French = <any>"French",
    Italian = <any>"Italian"
}

The shown TypeScript code is transpiled into the following JavaScript code:

(function (Language) {
    Language[Language["English"] = "English"] = "English";
    Language[Language["German"] = "German"] = "German";
    Language[Language["French"] = "French"] = "French";
    Language[Language["Italian"] = "Italian"] = "Italian";
})(exports.Language || (exports.Language = {}));
var Language = exports.Language;

With the shown changes to the enum, the output of the previous serialization code works as expected:

"{ language: "German" }"

Read more about this problem on this GitHub Issue page.