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

templates/admin/process.tmpl

@@ -1,7 +1,7 @@
 <h4 class="ui top attached header">
-	{{.i18n.Tr "admin.monitor.process"}}
+	{{.locale.Tr "admin.monitor.process"}}
 	<div class="ui right">
-		<a class="ui primary tiny button" href="{{AppSubUrl}}/admin/monitor/stacktrace">{{.i18n.Tr "admin.monitor.stacktrace"}}</a>
+		<a class="ui primary tiny button" href="{{AppSubUrl}}/admin/monitor/stacktrace">{{.locale.Tr "admin.monitor.stacktrace"}}</a>
 	</div>
 </h4>
 <div class="ui attached segment">