Which domain should I buy: domain.info, domain.biz, domain.eu, domain.net.pl, domain.org.pl. The site in one of these domains is going to be in Polish.
Depends on your area. But if that is business domain name I suppoose that worth to buy all existing domain name extensions for it. That is just recommended.
if your website is related with online business .biz is good for you otherwise you can buy any kind of domains.in my experience .com,.info,.net,.org domains are most popular so you can try to buy them.because this can help you in future for getting your website nice page rank in seo quickly.
If you would like to sell services in Poland then you should use .pl domain or else you should at first try .com, .biz or .co domain
I would always try and get the .com first, if not available and you want your site to rank up well on search engines and be seen globally then I would go .net next If it's only for Poland then .pl is a good bet.