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.
Your code looks right just do one thing provide the text domain of your current theme in the _e() function like below code :
<label for="user_login"><?php _e("Username or E-mail:",'textdomain') ?>