I reused the overlayed ../availablecolumns/language node and changed the jcr:title to "File Name" instead of creating a new one.
In overlayed asset.jspAdded the String filename = resource.getName(); inside the scriplet I replaced the <%= filename %> by <%= displayLanguage %> in the <td> element.
Everything is working perfectly including sorting when the content list is <=40 since it does the client-side sorting.
If the content list >40 then it calls the GET Api with ?sortName=language&sortDir=asc and get the results and populate the result but sorting not happens. except sorting everything is fine. Need sorting as well in the customized column. rest of the columns are sorting as expected.