I don’t know if anyone is getting this but even after resetting permissions, clearing cache, and doing a rebuild I am getting 404 for all en_us.js files in
/cache/jsLanguage. This is localhost on OSX. The files exist and have proper permissions.
Are you sure that permissions are correct? In addition to setting them via (OSX) shell have you updated them in config.php?
Has anything happened to the files recently?
Have you changed anything in your .htaccess files (you should also check the one in root folder)? In any case try Admin->Repair->.htaccess
Have you, by any chances moved the folder?
Is the variable site_url inside config set to the correct url?
Try moving the files temporarily to a backup folder then run Quick Repair and rebuild as well rebuild the JS.
(I recommend that you back everything up: files, folders and DB before changing things)