Idea for an app is quite nice. My kids would love it.

I just can't understand how can adult person be so traumatized by silly mistake in a coffee shop, so they will build an entire app to learn a language so this will not happen again.

I mean, I understand you made an error and you could not understand native speaker. Happens to me a lot of times with English and British people. But situation that you may not understand someone speaking foreign language abroad is expected. For me at least. How can you call it a humiliation? Just smile and ask politely to repeat because you do not understand. Point at you ear, which should be understand by everyone that you did not hear. Or look at register and check the price. Or just give them much more that you think it really cost and wait for a change. Awkwardly looking at your phone seems a bit rude.