Refactor i18n to locale (#20153)

* Refactor `i18n` to `locale`

- Currently we're using the `i18n` variable naming for the `locale`
struct. This contains locale's specific information and cannot be used
for general i18n purpose, therefore refactoring it to `locale` makes
more sense.
- Ref: https://github.com/go-gitea/gitea/pull/20096#discussion_r906699200

* Update routers/install/install.go

services/mailer/mail_repo.go

@@ -74,7 +74,7 @@ func sendRepoTransferNotifyMailPerLang(lang string, newOwner, doer *user_model.U
 		"Language":    locale.Language(),
 		"Destination": destination,
 		// helper
-		"i18n":      locale,
+		"locale":    locale,
 		"Str2html":  templates.Str2html,
 		"DotEscape": templates.DotEscape,
 	}