Support adding custom locales for admin UI

[DarkSuniuM]

What does it do?

Allows developer to extend packages/core/admin/admin/src/translations/languageNativeNames.js using languageNativeNames key admin config (src/admin/app.js)

Why is it needed?

Without this, extending translations for custom languages doesn't work.

How to test it?

Modify examples/getstarted/src/admin/app.js and put the following:

export default {
  config: {
    locales: ['fa'],
    languageNativeNames: {
      fa: 'فارسی',
    },
    translations: {
      fa: { 'Settings.profile.form.section.experience.interfaceLanguage': 'زبان رابط کاربری' },
    },
  },
  bootstrap() {},
};

Run the following command: yarn workspace getstarted develop --watch-admin

Open http://localhost:8000/admin/me in your browser, Set Interface Langauge to "فارسی", and Save.

The "Interface Language" title should change to "زبان رابط کاربری" which is defined in examples/getstarted/src/admin/app.js

[strapi-cla]

All committers have signed the CLA.

[strapi-bot]

This pull request has been mentioned on Strapi Community Forum. There might be relevant details there:

https://forum.strapi.io/t/unable-to-add-a-new-language-for-the-admin-ui/14251/7

[shestakov-vladyslav]

Good thing

[DarkSuniuM]

Good thing

This PR had some issues which I'll fix later

[pimpelsang]

So what happened to it?

[EmanuilGerganov]

How to add a new language to Strapi admin panel?

[AleksiupasSameSystem]

How to add a new language to Strapi admin panel?