First of all polish girls are not so different than girls from any other nationality. Why you are especially searching for a Polish Girl. If reason is some rumours you heard, they are not true.
I do not live in Poland, so internet seems a good way to communicate with polish girls.
Generally internet is not a good way to start a serious relationship, but there are exceptions and you may be lucky.
Could I expect a real-world relationship someday by using these sites, or it is all about fun/chatting for site's members and being outside Poland would mean that there is no chance for me to find a polish girl?
It is all up to you and how important for you to find a Polish Girl anything is possible.
What things should I pay attention when I try to find a polish girl using fotka or other polish social sites?
You should be carefull about scammers. You should not get attached to someone who you did not see. And if you think you really like someone that you find there it is better to meet in real life as soon as possible.
Are polish girls positive to migrate to other countries in european union, generally speaking?
Some or some not (Where are you Seanus ? :) )
If there is a good chemistry between a foreign guy and a polish girl, is there a hope for the polish girl to migrate outside Poland?
In most cases Ending up in Poland is also a possibility. Is it ok for you.