There is a plugin with other languages translations. I used poedit, online po editors and even plugins to update some strings in localization.
Here are sources:
<label for="user_login"><?php _e("Username or E-mail:") ?><br />
on line 911 in plugin's php file
#: ../other-includes/wp-security-rename-login-feature.php:561
#: ../other-includes/wp-security-rename-login-feature.php:911
msgid "Username or E-mail:"
msgstr "Имя пользователя или e-mail:"
in plugin's localization po file.
The name of locale files is correct. The files are correct. Other strings are fine end whean I edit them, the translation works.
I've changed string in php, then updated po from sources, edited translation, but nothing has happened.