constructor({ lang, isRTL }, l10n = null) {
    this.#lang = L10n.#fixupLangCode(lang);
    this.#l10n = l10n;
    this.#dir = (isRTL ?? L10n.#isRTL(this.#lang)) ? "rtl" : "ltr";
  }
NOTE: The L10n-implementations should use lowercase language-codes
      internally.
@implements {IL10n}