Masters of the Air show, Polish subtitles are not working

AirPlay ‘casting’ does make a difference: That would apply the subtitles as configured on the receiving device, when that receiving device (Apple TV box or smart TV or select third part streaming devices) has configuration for that. On supported third party devices, it should have Apple AirPlay Settings in the device settings. See if that has options for subtitles. Else see that device’s general subtitle settings.

It seems that I am not able to replicate your issue. When I select Polish subtitles, then I get Polish subtitles, for each of the nine episodes. While I can’t read Polish, I do recognize special characters (ą, ć, ę, ł, ń, ś, ź) specific to the language, and I did enter some sentences into Google Translate to have that recognize the language.

If this happens for you for a specific episode at a specific time stamp, then please specify that, in case I missed the precise part with an exception.