[i18n] ability to dynamically load translations for the app

# 🐞 bug report

### Affected Package
@angular/localize

### Is this a regression?
Yes, it's a regression from v8

### Description
In v8 was a very handy solution to load your translation data for development.
It's described on this documentation page https://v8.angular.io/guide/i18n#merge-with-the-jit-compiler

With the migration to v9 there's no corresponding solution that will allow to quickly switch locales during runtime for development. As far as I understand, the suggested replacement is to run `ng serve` with the desired locale every time.

A little bit background here. We have 2 applications, every with 4 languages. For production, we build it with AOT as suggested. For development we use JIT and load corresponding translation depending on the browser URL (German for `/de/` for example), this mimics prod behavior. So, to switch translation and check how the interface looks in a different language, you just need to pick up another item in the language menu. The page reloads at the same URL but with a different language. Starting from v9 it's not supported, so we would have to run `ng serve` every time for a new language and start from the home page for every language again.

What would be a suggested way repeat v8 behavior? I checked `loadTranslatios` but it accepts a key-value object, not a XLIFF data. Is any function that can transform from XLIFF to a format understandable for `loadTranslations`

## 🌍 Your Environment

**Angular Version:**

Angular CLI: 9.1.9

Node: 12.4.0

OS: darwin x64


Angular: 9.1.11

... animations, common, compiler, compiler-cli, core, forms

... language-service, localize, platform-browser

... platform-browser-dynamic, router

Ivy Workspace: Yes

Package                            Version

------------------------------------------------------------

@angular-devkit/architect          0.901.9

@angular-devkit/build-angular      0.901.9

@angular-devkit/build-ng-packagr   0.901.9

@angular-devkit/build-optimizer    0.901.9

@angular-devkit/build-webpack      0.901.9

@angular-devkit/core               9.1.9

@angular-devkit/schematics         9.1.9

@angular/cdk                       9.2.4

@angular/cli                       9.1.9

@angular/material                  9.2.4

@angular/material-moment-adapter   9.2.4

@ngtools/webpack                   9.1.9

@schematics/angular                9.1.9

@schematics/update                 0.901.9

ng-packagr                         9.1.5

rxjs                               6.5.5

typescript                         3.8.3

webpack                            4.42.0

@angular/localize とリンクされている v8 JIT i18n ドキュメントから始め、v9 loadTranslations API と、要求されている XLIFF および runtime-locale のワークフローを比較します。開発中にロケールを切り替えるためのサポートされている方法があり、XLIFF データの扱い方も含まれていれば完了です。

索引モデルが issue の本文から書いたものです。