Most EU citizens have the right to cast their vote in the European Parliament election in any other member state of the European Union. However, every country has their own set of rules governing the election process and it is worth learning them before you head to your nearest polling station. Start by checking the rules of your coutry of origin: How to vote from abroad.
If you reside in a foreign country and you are registered to vote there, you can either vote for candidates from your home country or the country of your residence. Remember that you can only choose one option and vote once.
Elections in Poland - how to register?
You can vote in Poland if you are a permanent resident of the country (you do not need to be a Polish citizen) and you are at least 18 on the day of the election. If you are not on the electoral roll or you have changed your address and want to vote in a different area, you should request to be added to the electoral register in your municipality, no later than 5 days before the election (it is advisable not to leave this task until the last minute). In order to do that, you need to submit a special application form to your district office (urząd dzielnicy/gminy). Visit the website of your town or city for the relevant document (for example, this application form applies to inhabitants of Warsaw).
European elections - basic information
European elections in Poland
Voting in Warsaw (general info, polling stations in Warsaw)