Hi,
There's no need to download it. Just make sure the language is installed in you Joomla installation and you have setup a content language for it. Once that is in place the language also becomes available in Acymailing.
To make changes to the translation go to Acymailing->Configuration->Languages and click on the edit icon for the language you would like to change. Once there you'll see the language strings. Copy the string you would like to change, scroll down to the bottom panel, paste your string, edit it and save it.
