I installed and configured dotClear2 from scratch. Now, there is no problem with mixed Polish and English entries because I set up two independent miniblogs on one installation. User language detection is now integrated to the website's system and it is based on cookies and browser settings. I made also nicer and more friendly theme.
And why not Wordpress? I don't like it. There are no real and friendly templates (yes, I love sweet chaos with mixed PHP and HTML). So far, I had an unpleasant opportunity to create from scratch a template for Wordpress and it was a terrible experience. I have been working with dotClear2 earlier and it is very easy to configure everything and convert default theme to website's design. Moreover, it is a nice piece of OOP code in PHP5. ;-)
Team Invenzzia hasn't got the official logo yet. This one on website is only a temporary solution.