Clean key & Update Ru lang

In the file `modules/system/lang/en/lang.php` I deleted a key `sendmail` at line [117](https://github.com/octobercms/october/blob/master/modules/system/lang/en/lang.php#L117) this duplicate line [108](https://github.com/octobercms/october/blob/master/modules/system/lang/en/lang.php#L108).