I don't speak Spanish well at all but I understand it. In most situations where I hear it I just ignore it/treat it as I would an English conversation. I don't think you need to clue people in, etc. If someone is speaking in public in any language they need to expect to be understood. But, I've also had situations where people that I'm going to have many repeated interactions with speak Spanish. In those cases, I do try at some point early on to comment about something/tell as story that allows me to tell them I understand the language without putting them on the spot/embarrassing them/interrupting a conversation. For example, my husband has a coworker who we see socially fairly often. He and his wife speak Spanish together.....not *usually* when others are around, but if they get a bit annoyed at each other they switch to Spanish to hash it out. I don't think the Spanish switch had to do with "hiding" the conversation from others but more so just the fact that when you're emotionally involved you gravitate to your first language. Anyways, the first time it happened I didn't say anything....but the next time we were out together I made a point to tell some story that included me explaining how I know Spanish, just so they'd know in the future.... I've also had a similar situation with coworkers at work that I worked closely with, and handled it similarly.