Tadzio and HIs Family Speaking Fluent Polish?

Tadzio and his family are only heard speaking from a distance, and that only in a few scraps of conversation. Now, as Björn Andrésen and the others were heard speaking French sometimes, I don't find it unusual that they might all have been more or less fluent in that language. (French was,, after all, the first foreign language that members of the Polish upper-class would have learned.) However, I also hear several phrases in Polish, which of course was supposed to be their native tongue. I find it hard to believe that this non-Polish cast would actually speak Polish--and rather fluently at that. Does anyone know whether these phrases, heard from afar, were dubbed or not?