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/routes.go

@@ -57,7 +57,7 @@ func installRecovery() func(next http.Handler) http.Handler {
 					store := dataStore{
 						"Language":       lc.Language(),
 						"CurrentURL":     setting.AppSubURL + req.URL.RequestURI(),
-						"i18n":           lc,
+						"locale":         lc,
 						"SignedUserID":   int64(0),
 						"SignedUserName": "",
 					}