在Angular 4中改变语言后如何改变字符串？

{
  "hello": "Hello",
  "world": "World"
}

{
  "hello": "你好",
  "world": "世界"
}

import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { TranslateModule, TranslateLoader } from '@ngx-translate/core';
import { TranslateHttpLoader } from '@ngx-translate/http-loader';
import { HttpClient, HttpClientModule } from '@angular/common/http';

export function HttpLoaderFactory(http: HttpClient) {
  return new TranslateHttpLoader(http, './assets/i18n/', '.json');
}

@NgModule({
  imports: [
    BrowserModule,
    HttpClientModule,
    TranslateModule.forRoot({
      loader: {
        provide: TranslateLoader,
        useFactory: HttpLoaderFactory,
        deps: [HttpClient]
      }
    })
  ],
  declarations: [AppComponent],
  bootstrap: [AppComponent]
})
export class AppModule { }

<h1>{{ 'hello' | translate }}</h1>
<p>{{ 'world' | translate }}</p>

import { Component } from '@angular/core';
import { TranslateService } from '@ngx-translate/core';

@Component({
  selector: 'app-root',
  template: `
    <select (change)="changeLanguage($event.target.value)">
      <option value="en">English</option>
      <option value="zh">中文</option>
    </select>
    <h1>{{ 'hello' | translate }}</h1>
    <p>{{ 'world' | translate }}</p>
  `
})
export class AppComponent {
  constructor(private translate: TranslateService) {
    translate.setDefaultLang('en');
  }

  changeLanguage(lang: string) {
    this.translate.use(lang);
  }
}