JHShop by default ships with the english language files, should you wish to add additional languages you can do it by following the following steps:

1) copy components/com_jhshop/language/english to components/com_jhshop/language/yournewlanguage

2) rename components/com_jhshop/language/yournewlanguage/english.php to yournewlanguage.php

Repeat the above with the administration area files

You can now translate the files into your language as needed.

In the JHShop administration area under settings ->localisation->languages click the insert button and fill out the form with the information as requested.

If all you want to do is change text in certain areas of the site, you can edit these same language files with the newer text.