In your skins/yourskin/_main.php file find something like

<h3>Categories</h3>

and change it to

<h3><?php echo T_('Categories') ?></h3>

Do the same for all of the 'stock' words and phrases that appear in English. Translating will depend on if the localization file for your language has those words and phrases translated, but most of the sidebar stuff should be translated in most language packs.

Heh,
I know, I translated a lot...
But I don't have

<h3>Categories</h3>

I have only

<?php
// -------------------------- CATEGORIES INCLUDED HERE -----------------------------
	// Call the Categories plugin:
	$Plugins->call_by_code( 'evo_Cats', array( // Add parameters below:
) );
// -------------------------------- END OF CATEGORIES ----------------------------------
?>

PS.
I still don't know where change comments. It isn't in _main.php

Oh yeah I forgot categories are done by a plugin now. The plugin (plugins/_categories.plugin.php) has the code to do the translation, so the only thing I can think of is that it's your locale file (locales\pl_PL\LC_MESSAGES) doesn't have a translation for that particular bit.

I guess you already know about how to update your language file, but if you haven't been doing it the way http://manual.b2evolution.net/Localization eventually talks about you might want to. IF I understand correctly, by working with the master .PO file you'll be able to see all the words and phrases that need translation to your particular language. Being an English user it's not something I've ever had to learn that much about, so I'm sorry but I can't help much more than that.