As of Angular2 RC6, you can set default locale in your app module, by adding a provider:

@NgModule({
  providers: [
    { provide: LOCALE_ID, useValue: "en-US" }, //replace "en-US" with your locale
    //otherProviders...
  ]
})

The Currency/Date/Number pipes should pick up the locale. LOCALE_ID is an OpaqueToken, to be imported from angular/core.

import { LOCALE_ID } from '@angular/core';

For a more advanced use case, you may want to pick up locale from a service. Locale will be resolved (once) when component using date pipe is created:

{
  provide: LOCALE_ID,
  deps: [SettingsService],      //some service handling global settings
  useFactory: (settingsService) => settingsService.getLanguage()  //returns locale string
}

Hope it works for you.

Solution with LOCALE_ID is great if you want to set the language for your app once. But it doesn’t work, if you want to change the language during runtime. For this case you can implement custom date pipe.

import { DatePipe } from '@angular/common';
import { Pipe, PipeTransform } from '@angular/core';
import { TranslateService } from '@ngx-translate/core';

@Pipe({
  name: 'localizedDate',
  pure: false
})
export class LocalizedDatePipe implements PipeTransform {

  constructor(private translateService: TranslateService) {
  }

  transform(value: any, pattern: string = 'mediumDate'): any {
    const datePipe: DatePipe = new DatePipe(this.translateService.currentLang);
    return datePipe.transform(value, pattern);
  }

}

Now if you change the app display language using TranslateService (see ngx-translate)

this.translateService.use('en');

the formats within your app should automatically being updated.

Example of use:

<p>{{ 'note.created-at' | translate:{date: note.createdAt | localizedDate} }}</p>
<p>{{ 'note.updated-at' | translate:{date: note.updatedAt | localizedDate:'fullDate'} }}</p>

or check my simple "Notes" project here.

With angular5 the above answer no longer works!

The following code:

app.module.ts

@NgModule({
  providers: [
    { provide: LOCALE_ID, useValue: "de-at" }, //replace "de-at" with your locale
    //otherProviders...
  ]
})

Leads to following error:

Error: Missing locale data for the locale "de-at".

With angular5 you have to load and register the used locale file on your own.

app.module.ts

import { NgModule, LOCALE_ID } from '@angular/core';
import { registerLocaleData } from '@angular/common';
import localeDeAt from '@angular/common/locales/de-at';

registerLocaleData(localeDeAt);

@NgModule({
  providers: [
    { provide: LOCALE_ID, useValue: "de-at" }, //replace "de-at" with your locale
    //otherProviders...
  ]
})

Documentation