Hi Piotr,
like I said, I tried to reproduce the behaviour with adding a random string somewhere in the templates, and things worked as expected.
Please make sure you've restarted your Django runserver after updating translation files, and the MO files are based on the current PO files. I'd also recommend pulling in up-to-date translations from Transifex.
If you can then still reproduce the error in the sandbox, please file a bug report and I'm happy to look into it further.
Regards,
Maik