[[AdvSearchForm? &method=`POST` &myLang=`fa`]]
This snippet is for outputting the search form, it has nothing to do with the results.
To output the form in your language, you need to edit the language tags in your chunk.
If your chunk is called faSearchForm then the snippet call will look like this:
[[AdvSearchForm? &method=`POST` &tpl=`faSearchForm` &landing=`80`]]
And should output the search form in your language and show results on a page with an id of 80.
To show results in Farsi, you can use the AdvSearch snippet on the results page (80 on the above example), and restrict the search to the Farsi folder.
So if the folder Id is 30, you can use:
[[!AdvSearch? &ids=`[[!GetIds? &ids=`c30`]]]]
This example also uses the getIds snippet to get all the children of the resource with the id of 30.