Add option to set language in admin user view (#28449)

![image](https://github.com/go-gitea/gitea/assets/24977596/be7e3f92-af3f-4628-b4ed-abf6439687f3)
`/admin/users/<UserID>/edit`


![image](https://github.com/go-gitea/gitea/assets/24977596/906af0dd-cceb-4ed9-9cd9-32c71ae1bf71)
`/admin/users/<UserID>`

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*Sponsored by Kithara Software GmbH*

services/forms/admin.go

@@ -41,6 +41,7 @@ type AdminEditUserForm struct {
 	Password                string `binding:"MaxSize(255)"`
 	Website                 string `binding:"ValidUrl;MaxSize(255)"`
 	Location                string `binding:"MaxSize(50)"`
+	Language                string `binding:"MaxSize(5)"`
 	MaxRepoCreation         int
 	Active                  bool
 	Admin                   bool