a lawyer so I can sue my ex for slander?
Almost impossible to do unless unfortunately it's been proven beyond any doubt that she'd lied. That generally means her having admitted she'd lied and been already prosecuted for attempting to pervert the cause of justice. Also it isn't slander for her to report something to the police or a therapist - even if that report is pure fiction.This happens far more often than you'd think.
Instead, I got the stamp in Germany
If there was a European Arrest Warrant, (Poland very much overuses those) you would have been arrested already in Germany. Those warrants can only be issued if there are already criminal charges by a prosecutor (not just a complaint or an investigation).
Poland probably dont have statute of limitations.
Poland has a statute of limitations and also a reasonably high burden of proof - it isn't just one person's word against another as it is in many English-speaking countries.
I actually plan to go back in Poland. Would that also be a problem?
You need to find out if charges have actually been filed by a prosecutor - your country's embassy in Warsaw is probably a good point of contact.