feat: add language selector component to header

[andrey-canon]

Description

This adds a new language selector to the learning(sed by learning) header and desktop(used by learner-dashboard) header

Issue # 1474
Migration pr of #6 and #9

How to test

1. Checkout the required branches frontend-component-header and frontend-platform
2. Set both dependencies as editable in your MFE,e.g

module.exports = {
    /*
    Modules you want to use from local source code.  Adding a module here means that when this app
    runs its build, it'll resolve the source from peer directories of this app.
  
    moduleName: the name you use to import code from the module.
    dir: The relative path to the module's source code.
    dist: The sub-directory of the source code where it puts its build artifact.  Often "dist".
    */
    localModules: [
        { moduleName: '@edx/frontend-platform', dir: '../frontend-platform', dist: 'src' },
        { moduleName: '@edx/frontend-component-header/dist', dir: '../frontend-component-header', dist: 'src' },
        { moduleName: '@edx/frontend-component-header', dir: '../frontend-component-header', dist: 'src' },
    ],
};

3. Add the following settings to your local environment

MFE_CONFIG["SITE_SUPPORTED_LANGUAGES"] = ["en", "ar"]
MFE_CONFIG["ENABLE_HEADER_LANG_SELECTOR"] = True

4. Download your MFE messages
   export PATH="$(pwd)/node_modules/.bin:$PATH"
make OPENEDX_ATLAS_PULL=true ATLAS_OPTIONS="--repository=nelc/futurex-translations --revision=open-release/redwood.master" pull_translation
5. Go to your mfe an check the language selector behavior

2026-08-11.14-34-04.mp4

[johanseto]

Working in my local env

Screencast.from.13-08-26.09.40.37.webm