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

routers/install/install.go

@@ -69,7 +69,7 @@ func Init(next http.Handler) http.Handler {
 			Render:  rnd,
 			Session: session.GetSession(req),
 			Data: map[string]interface{}{
-				"i18n":          locale,
+				"locale":        locale,
 				"Title":         locale.Tr("install.install"),
 				"PageIsInstall": true,
 				"DbTypeNames":   dbTypeNames,