Not quite, you should not need to modify any of the files in the plugins directory.
The language string can be found in administrator/language/en-GB/en-GB.plg_system_jat3.ini
#_________________________Frontend-language___________________________#
YOU ARE HERE=You are here
SKIP TO CONTENT=Skip to content
HOME PAGE TITLE=Welcome to the Frontpage
If you look at any language string in the code that has
JText::_(
Means look in the language file. In this case using a tool such as wingrep (http://www.wingrep.com/) allows you to search through folders/files for any string and a quick search for “you are here” brings up all locations with that string. SO if you struggle to find any language strings in future try wingrep.
As for modifying files in the jat3 plugin it is much better to copy that block to the templates directory. Any of the php files in the jat3 plugins blocks directory can be copied over to the templates blocks directory and they will take precedent. Doing this means you can update your plugin without having to re-apply your modifications.