With a default Joom!Fish installation you can translate all components, modules etc. from the core. To translate a 3rd party extension you need to have extra files on your system. Those files are called 'content elements', and they're just like the ones for menus, articles, banners etc.
So first thing to do is to get the file. Those files aren't stored in a central repository. (There are plans to create such a repository here, but this is future talking...). Some sources are listed here. The advise is to end up with the latest version of the content elements.
Then we need to install the content elements. I'll show this with an example. I'm going to enable Joom!Fish to translate the events from JEvents.
So first I search for the content element files, which are convienently located on the project's website. Then I'll download the file and save it on my computer
Next I go to my Joomla! administrator backend to Components > Joom!Fish > Content Elements. There is an overview of all installed Content Elements. You can install new Content Elements by chosing the 'Install' button on the top bar.
Then you see an area where you can upload your just downloaded content element file. As you see here I do this for JEvents (which is an .xml file)
I press install and voilà, I get the message that it's uploaded succesfully.
Then I check my translation overview under Components > Joom!Fish > Translations and I can select JEvents from the content elements drop-down list.