How do I 'ng serve' the original app after localizing it with @angular/localize?

I was playing with Angular's localization @angular/localize and after configuring it to translate the app to different language I tried to serve the default version of it using 'ng serve' but I get this error:

An unhandled exception occurred: The development server only supports localizing a single locale per build.

I tried it with different port based on this: How to serve different Angular locales during development using ng serve? but to no avail.

For the translated version everything works using 'ng serve --configuration=fr'. Builds work correctly creating 'en-US' and 'fr' folders in dist/App folder.

When I removed the inlineLocales.size condition in dev-server everything worked fine, no errors:

if (i18n.shouldInline && tsConfig.options.enableIvy !== false) {
    //if (i18n.inlineLocales.size > 1) {
    //    throw new Error('The development server only supports localizing a single locale per build');
    //}
    await setupLocalize(i18n, browserOptions, webpackConfig);
}

So it may be Angular-devkit issue.

I tried to work around by creating new "en-US" locale but got this error:

An unhandled exception occurred: An i18n locale ('en-US') cannot both be a source locale and provide a translation.

After using just 'en' instead of 'en-US' and creating new translation file with <source> and <target> translation being the same it works using 'ng serve --configuration=en' but it looks like a wrong solution.

Here is my angular.json:

{
  "$schema": "./node_modules/@angular/cli/lib/config/schema.json",
  "version": 1,
  "newProjectRoot": "projects",
  "projects": {
    "App": {
      "projectType": "application",
      "i18n": {
        "sourceLocale": "en-US",
        "locales": {
          "fr": "src/locale/messages.fr.xlf"
        }
      },
      "schematics": {
        "@schematics/angular:component": {
          "style": "scss"
        },
        "@schematics/angular:application": {
          "strict": true
        }
      },
      "root": "",
      "sourceRoot": "src",
      "prefix": "app",
      "architect": {
        "build": {
          "builder": "@angular-devkit/build-angular:browser",
          "options": {
            "localize": true,
            "outputPath": "dist/App",
            "index": "src/index.html",
            "main": "src/main.ts",
            "polyfills": "src/polyfills.ts",
            "tsConfig": "tsconfig.app.json",
            "aot": true,
            "assets": ["src/favicon.ico", "src/assets"],
            "styles": ["src/styles.scss"],
            "scripts": []
          },
          "configurations": {
            "production": {
              "fileReplacements": [
                {
                  "replace": "src/environments/environment.ts",
                  "with": "src/environments/environment.prod.ts"
                }
              ],
              "optimization": true,
              "outputHashing": "all",
              "sourceMap": false,
              "extractCss": true,
              "namedChunks": false,
              "extractLicenses": true,
              "vendorChunk": false,
              "buildOptimizer": true,
              "budgets": [
                {
                  "type": "initial",
                  "maximumWarning": "500kb",
                  "maximumError": "1mb"
                },
                {
                  "type": "anyComponentStyle",
                  "maximumWarning": "2kb",
                  "maximumError": "4kb"
                }
              ]
            },
            "fr": {
              "localize": ["fr"]
            }
          }
        },
        "serve": {
          "builder": "@angular-devkit/build-angular:dev-server",
          "options": {
            "browserTarget": "App:build"
          },
          "configurations": {
            "production": {
              "browserTarget": "App:build:production"
            },
            "fr": {
              "browserTarget": "App:build:fr"
            }
          }
        },
        "extract-i18n": {
          "builder": "@angular-devkit/build-angular:extract-i18n",
          "options": {
            "browserTarget": "App:build"
          }
        },
        "test": {
          "builder": "@angular-devkit/build-angular:karma",
          "options": {
            "main": "src/test.ts",
            "polyfills": "src/polyfills.ts",
            "tsConfig": "tsconfig.spec.json",
            "karmaConfig": "karma.conf.js",
            "assets": ["src/favicon.ico", "src/assets"],
            "styles": ["src/styles.scss"],
            "scripts": []
          }
        },
        "lint": {
          "builder": "@angular-devkit/build-angular:tslint",
          "options": {
            "tsConfig": [
              "tsconfig.app.json",
              "tsconfig.spec.json",
              "e2e/tsconfig.json"
            ],
            "exclude": ["**/node_modules/**"]
          }
        },
        "e2e": {
          "builder": "@angular-devkit/build-angular:protractor",
          "options": {
            "protractorConfig": "e2e/protractor.conf.js",
            "devServerTarget": "App:serve"
          },
          "configurations": {
            "production": {
              "devServerTarget": "App:serve:production"
            }
          }
        }
      }
    }
  },
  "defaultProject": "App"
}

How to reproduce: The app is generated using Angular CLI 10.0.0 and Angular 10.0.1 and then adding @angular/localize by 'ng add @angular/localize'. In app.component.html is single <h1 i18n="@@app.h1">Hello</h1>. Using the angular.json above.

In "options", you must set "localize" to false:

"architect": {
    "build": {
      "builder": "@angular-devkit/build-angular:browser",
      "options": {
        "localize": false,

and build again:

ng build