well established and diverse throughout Europe
There are quite a lot of foreign owned companies in Poland that are not as good to work for as you might imagine because the Polish offices are managed by locals and they have their own way of doing things. However, it's a bit of a lottery and it could suit you quite well.
Polish & Slavic Federal Credit Union here in America could act as some kind of notary
If the bank wants a USA credit history from you they may specify which agency it should be from. There is no harm in having a bank reference or credit reference but as another poster mentioned, it won't carry that much weight. Things are different in Poland. The bank is not that interested in your past, they care about your present. How much are you earning, what kind of employment contract do you have, what are your outgoings, any other loans to service. The only thing they want to be sure of is that you're not currently wanted for an unpaid debt. Otherwise, they're really not impressed with a faultless credit history, a good record of saving etc. It won't influence their decision to grant you a mortgage.
I don't know how it works in the USA, but you should also know that although you can have a meeting to discuss your requirements and get a 'yes we would
give you a mortgage' in theory, you can't actually
apply for one until you find a property you want. In Ireland, where I'm from you apply and get approval for a certain amount, then when you find a house you draw down the mortgage. In Poland you have to wait until you find a house and then
you apply for a mortgage for that particular property. It takes about six weeks to process the application. Oh and the mortgage has to be joint with your wife unless you sign a 'separation of property' agreement. That means you will be making all your financial affairs separate, not just the ownership of the house.
Poland is a very different kettle of fish legally to the USA because your laws mostly originate with the English Common Law system (and a bit of constitutional law) while Poland has the Roman law system, more common on the continent and a complete pain in the nether portions, in my opinion. But that's a discussion for another day!