Calling loadTranslations() in either main.ts or in a resolved Promise does not work, at least in Angular 9 rc12. The function has to be called synchronously in polyfills.ts, so the translations also have to be available as a JS object, i.e. you can't fetch or XHR the JSON.
You could require() the JSON file in polyfills.ts, but then the JSON for all the languages will be in the bundle. (Try it and search for each JSON in polyfills.js.)
Alternatively, save each translation as a .js file and add a <script>
tag to load only the desired translations.