Ok, so how is it going to work?

Assume I just logged first time - im from Poland, so will game place me together with Polish players? Or it will be just random choice?

What if I decide to join French guild?

I guess I will automatically be placed in the same word my guild is, but then assume I want to raid with few friends from Poland? Then when I join raid am I placed on world where raid leader is located?

Or does it matter? May be interaction is planned only between guild/raid/team, but for AvAvA battles it is not so - all players in one instance must interact.

I believe that is an older quote, and they haven't said much recently to support that older plan. It would seem that the current model is that there will be multiple megaservers around the planet, and that you may choose to select one at a time. You may play with anyone on that megaserver and all characters you create will be on that megaserver. Further details have not been mentioned to my knowledge. If you would want to play with friends, you may join the same megaserver, namely the EU megaserver, then join with your friends in either a group or guild and you will be phased together while playing. This is my understanding on how joining with friends will function.