Fix extension translation system

Despite that documentation says that locales should be formatted with underscore, formatting them with hyphen is also supported in Firefox. This commit removes the underscore conversion, as it makes it easier to manage translations through Crowdin.

extension/src/utils/i18n.js

@@ -23,7 +23,7 @@ export async function getCurrentLocale () {
  * @returns {string[]}
  */
 export function getAllLocales () {
-  return Object.keys(messages).map(code => code.replace('_', '-'))
+  return Object.keys(messages)
 }
 
 /**
@@ -83,7 +83,7 @@ function parseICUPlural (message, count, locale) {
  * @returns {Promise<string|undefined>}
  */
 async function getMessageForLocale (key, substitutions, count, locale) {
-  const message = messages?.[locale.replace('-', '_')]?.[key]
+  const message = messages?.[locale]?.[key]
   if (!message) return
 
   let content = message.message