One of the messages in .po file doesn’t show up

I’ve changed a file wp-content/languages/pl_PL.po

#: bfa_header.php
msgid "notice"
msgstr "napis"

msgid "Online Information Website"
msgstr "Internetowy Portal Informacyjny"

msgid "something"
msgstr "coś"

I’ve added the second pair. It works for another language, but doesn’t work for this one. All other messages are working. What might be the problem?

Looks like you may not be able to just add a pair to your .po file, without first adding it to the wordpress.pot file: http://codex.wordpress.org/Translating_WordPress#gettext_files. You’ll also need to compile your .po file into a .mo (Machine Object) file in order for your changes to take effect.

I’m not certain that will work, but it should be a good start.