when you want to translate a dot js file that haven't a function qa_lang() , you must convert all the messages in js variables and set them with the QA function related to content ( $qa-content ) :
writing this in php
$qa_content['script_var']['js_hello'] = qa_lang('myplugin/js_hello') ;
will produce in header this js line script , assuming the translation is provided in the appropriate lang file :
var js_hello = 'Bonjour la compagnie' ;
Then, you can safely replace 'Bonjour la compagnie' in the js file by js_hello and be sure that your translation in any language will be well handled.
See qa-ajax.js of the dbsave plugin ( or qa-chat2.js of the chat hack which implements the chat translation )