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/web/base.go

@@ -139,7 +139,7 @@ func Recovery() func(next http.Handler) http.Handler {
 					store := dataStore{
 						"Language":   lc.Language(),
 						"CurrentURL": setting.AppSubURL + req.URL.RequestURI(),
-						"i18n":       lc,
+						"locale":     lc,
 					}
 
 					user := context.GetContextUser(req)