Use BrowserProcessImpl::GetApplicationLocale for

`BraveContentBrowserClient::GetApplicationLocale` because the result of
`extension_l10n_util::CurrentLocaleOrDefault` will be normalized by
`l10n_util::NormalizeLocale` which means turning en-US to en_US

`chrome.i18n.getUILanguage` uses `RenderThread::GetLocale()` which reads
from switch `--lang` appended when renderer process created and the
value is reading from `ContentBrowserClient::GetApplicationLocale`

fix brave/browser-laptop#11624

Auditors: @bridiver, @bbondy

brave/browser/brave_content_browser_client.cc

@@ -582,7 +582,7 @@ std::string BraveContentBrowserClient::GetApplicationLocale() {
   if (BrowserThread::CurrentlyOn(BrowserThread::IO)) {
     return io_thread_application_locale.Get();
   } else {
-    return extensions_part_->GetApplicationLocale();
+    return g_browser_process->GetApplicationLocale();
   }
 }