Tongue of the Sun and Moon

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.

You can activate or deactivate this feature at will, no action required.

When you use this feature to speak with a creature, the language you speak does not change in the creature's hearing, yet it understands. When the creature would not understand you without the use of this feature, it must make an Investigation or Perception check against your Ki / Monk's Focus DC to notice that you are speaking words it would not normally understand. Whether the check for the creature is passive or active is up to the DM and the context, but no action is required to make it.