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.
Important : If a plugin is updated, the entire plugin folder is emptied and the new files are put into that folder, which means that any custom language files you may have will be lost so you must back them up before upgrading plugins.