Do not delete installed language during plugin update

After I have install BP Group Calendar plugin, I go to its "languages" folder via ftp and copy po/mo files with my language constants. Then, a new plugin update comes, I upgrade it via my wp admin console - and my languages constants turns back to default english, so I must to open my ftp client again and manually install mo/po files again. The question is - how to upgrade plugin and do not to copy mo/po again and again?