**Yes, part of it**

Tongue of the Sun and Moon is two features under one ability name. You have the **ability** to understand all spoken languages, and other creatures **are capable** of understanding your speech. If you want to choose to NOT understand a creature's speech, you are allowed to not understand.

The way I see it, the second sentence can be interpreted in two ways: 1) "Any creature that is capable of understanding a language **is capable** of understanding what you say, period," and 2) "Any creature that is capable of understanding a language **may choose to** understand what you say." Whichever interpretation of the wording the GM goes with, it is not the monk's choice whether other creatures are able to understand him.