I am a white American, but I have learned Spanish as a second language. Sometimes, I will end up in an enclosed area (perhaps a waiting room or an elevator), and the only people there are me and a group of two or more Hispanic people who are talking to each other in Spanish.
Of course, I don't pay any more attention to their conversation than I would to a conversation in English that I wasn't part of; in fact, I understand less of it, because I still have to pay fairly close attention to understand Spanish. However, I inevitably do understand some parts of it.
Since I'm not Hispanic, people might assume that I don't know Spanish, and think that they have more privacy than they really do. My concern is that they might talk to each other about sensitive topics that they wouldn't want a stranger hearing about, assuming that I won't understand what they are saying.
How can I politely indicate that I know Spanish, so I do understand parts of their conversation? Should I even do that?
Simply saying something like "¡Hola! Yo hablo español" ("Hello! I speak Spanish") might be awkward, as it could sound like I'm saying that I found something they said offensive, when that's not true at all.
I could also just say "¡Hola! ¿Cómo están ustedes?" ("Hello! How are you?"), but it seems rude to interrupt a group's conversation to ask how they are doing.
Is there a better way to indicate that I speak Spanish?