how to know the un used language strings ?

Long way would be to search your codebase for each key of the language files $lang array.

Like if you had $lang['welcome_message'] = 'hello';

Use the IDE's search function to search for "welcome_message" for all files in /application.

A shorter, but more complicated, way would be to create a script that would include each lang file, run an array_keys() on it just to get the keys, and then the script would go through all of your files in /application using file_get_contents() or something and search for each key in the string (file). Once a key is found remove/unset it from the array that you originally stored the array_keys() in. What you have left would be the unused lang keys.