[api-minor] Move to Fluent for the localization (bug 1858715)

- For the generic viewer we use @fluent/dom and @fluent/bundle
- For the builtin pdf viewer in Firefox, we set a localization url
  and then we rely on document.l10n which is a DOMLocalization object.

web/interfaces.js

@@ -179,14 +179,14 @@ class IDownloadManager {
  */
 class IL10n {
   /**
-   * @returns {Promise<string>} - Resolves to the current locale.
+   * @returns {string} - The current locale.
    */
-  async getLanguage() {}
+  getLanguage() {}
 
   /**
-   * @returns {Promise<string>} - Resolves to 'rtl' or 'ltr'.
+   * @returns {string} - 'rtl' or 'ltr'.
    */
-  async getDirection() {}
+  getDirection() {}
 
   /**
    * Translates text identified by the key and adds/formats data using the args