chore(i18n): refactor and cleanup (#805)

* checkpoint

* finish

* docs

quartz/cfg.ts

@@ -1,5 +1,6 @@
 import { ValidDateType } from "./components/Date"
 import { QuartzComponent } from "./components/types"
+import { ValidLocale } from "./i18n"
 import { PluginTypes } from "./plugins/types"
 import { Theme } from "./util/theme"
 
@@ -39,9 +40,12 @@ export interface GlobalConfiguration {
   /**
    * Allow to translate the date in the language of your choice.
    * Also used for UI translation (default: en-US)
-   * Need to be formated following the IETF language tag format (https://en.wikipedia.org/wiki/IETF_language_tag)
+   * Need to be formated following BCP 47: https://en.wikipedia.org/wiki/IETF_language_tag
+   * The first part is the language (en) and the second part is the script/region (US)
+   * Language Codes: https://en.wikipedia.org/wiki/List_of_ISO_639_language_codes
+   * Region Codes: https://en.wikipedia.org/wiki/ISO_3166-1_alpha-2
    */
-  locale?: string
+  locale: ValidLocale
 }
 
 export interface QuartzConfig {