They do this so the Polish smugglers won't be able to understand them. This was a good idea, particularly for the Jews relying on Yiddish language, in order to convey secrets among themselves. The Jews have no trust to the smugglers in front of them, and so they use the language to have an advantage.
Historical Background
The history of the Jews in Poland dates back over 1,000 years. Many children were smuggled to Poland, where the conscription of Jews did not take effect until 1844.