[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

Bắt đầu với @angular/localize và tài liệu v8 JIT i18n được liên kết; so sánh API v9 loadTranslations với quy trình XLIFF và runtime-locale được yêu cầu. Công việc được xem là hoàn tất khi có một phương pháp được hỗ trợ để chuyển đổi locale trong quá trình phát triển, bao gồm cả cách xử lý dữ liệu XLIFF.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.