Of course the Polish police would had enough suspicion for the offence of theft (using UK definition).
However, as soon as the information changes, so does the authority to detain. Even without questioning the OP using an official interpreter, its possible for them to have some doubts that elements of the offence are incomplete. So the justification to detain, just because that had trouble sourcing an interpreter is suspect.
Even though the OP is a foreign national, he should of been given bail, to attend the police station when they have all things available, IMO.
OP should of been given access to legal advice, someone informed of his arrest, and the local British Consulate informed.
@Gingerbeard were you given these rights? As far as I've read, Police authorities (Polish) are signed up to these conventions.