When plugins are installed they are installed to the /jomres/core-plugins directory. Plugins are only ever supplied with an English language file, in a sub-directory off the remote plugins directory, so for example if you've installed the Jintour plugin, then the language file will be /jomres/core-plugins/jintour/language/en-GB.php. The same goes for any template files. If you want a new language file for the plugin, you can create a new file in that directory, so if you want to translate the plugin to German you'd copy en-GB.php to /jomres/core-plugins/jintour/language/de-DE.php and then translate the contents of that file.
You should not need to make your own language files for plugins. If you need to translate a specific label or labels the easiest way to do this is to visit the "Translate Lang File Strings" menu option in the administrator area and use the browser's search feature to search for the string in that page.
