en_RU (Russian) language added and updated to include latest strings
https://github.com/nurkamol/simpleinvoices/blob/dfa95db20301381d25d84f02970dbbc21e1e2f06/lang/ru_RU_lang.php
https://github.com/nurkamol/simpleinvoices/blob/45aa9b7c29867c990b8bbd315caf708b4a6df83b/lang/ru_RU_info.xml