The rules for the monk ability Tongue of the Sun and Moon state:
Starting at 13th level, you learn to touch the ki of other minds so that you understand all spoken languages. Moreover, any creature that can understand a language can understand what you say.
The text says that this ability is something you learn to do, not something that simply happens to you, so it's not necessarily always on. Further, it seems logical to say that this is something you would do actively. That is, you could choose not to touch someone's mind, thereby excluding them from understanding you. You could also choose not to understand someone yourself, just in case they're telling the funniest joke in the world, or using a subliminal trigger.
Assuming you can choose not to touch someone's mind, let's reason further. Suppose you're speaking and don't know that there is someone listening in nearby. Would they understand you, i.e. is the default that you are or aren't touching someone's mind? Similarly, if you are speaking to a large group of people, would you have to focus on all of them at once for them to understand you? Would this take effort? Is there a range?
My questions are:
- Can you exclude some characters from understanding you while allowing others to do so?
- Can you decide not to understand someone who is speaking?
Basically, these boil down to: Can you choose not to touch someone's mind?
Then, assuming that the answer to the above is yes:
- Do you have to know someone is listening in order for them to understand you?
- Is there a limit to the number of people you can affect? A range?