You'd have to ask individual plugin authors to include their translation with their plugins and load them at initialization time. calibre itself cannot keep translations for third party plugins.
Essentially what a plugin developer has to do is create a .po file from the strings in his plugin and upload it to launchpad/transifex or whatever. Then distribute the .mo files containing the translated strings inside the plugin zip file.
If you can get some plugin authors who are willing to do that, I will be happy to add some functionality to calibre to make it easy for them to load the .mo files from the plugin zip file.
|