Is there a way to download this language pack outside of the program? I looked for "Pirate English" under the language packs, but it is not there.
Why would you need to download outside of the program ?
The link you gave isn't to language packs but full installation packs in different languages.
You should be able to do it through the avastUI.
If it isn't in the drop down list of the avast Settings, Language, then you will have to click the Install additional languages, select Pirate Talk from the list and install it.