He managed to use it once when we were living in Kuwait and he had to arrange for my sister to get baptized - he knew that there were two missionaries visiting the local Cathedral - one Spanish and the other Polish but as he didn't know who was who, he started the conversation in Latin and of course got an appropriate response.
Also he figured it would be rude.
IIRC the wider context is that my dad, not knowing where the priests were at that moment and having no one to ask in the vicinity, went to a confessional as he saw someone there.
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.
Could it be he just wanted the child baptized, no matter if it was Spanish or Polish priest? Then, starting with Latin is the way to get a priest's attention.
Oh look at me, I'm being rude here - talking in Polish. Because that eases the convo.