Finally, I found the Solution.
it was in the way I call the function:
Language x;
x = new Language();
x.LanguageID = int.Parse(txt_LanguageID.Text);
x.Name = txt_Name.Text;
x.Lang = txt_Lang.Text;
x.IsActive = bool.Parse(ddl_IsActive.SelectedValue);
x.CreationDate = lbl_CreationDate.Text;
x.LastModifiedDate = DateTime.Now.ToShortDateString() + "";
RepLanguage.Update(x);
}</pre>