Both priests are assumed to understand Latin. The situation they want to avoid is starting up a conversation with someone in a language they don't know.
So the options are:
1) Use Polish, with the risk of being rude if you happen to speak to the Spanish priest
2) Use Latin and ask "do you speak Polish?", and both the Polish and the Spanish priest will understand.
You could of course argue that it's not so very rude to accidentally try to strike up a conversation with someone in a language they don't know, but apparently Tade0's father thinks it is.
So the options are:
1) Use Polish, with the risk of being rude if you happen to speak to the Spanish priest
2) Use Latin and ask "do you speak Polish?", and both the Polish and the Spanish priest will understand.
You could of course argue that it's not so very rude to accidentally try to strike up a conversation with someone in a language they don't know, but apparently Tade0's father thinks it is.