This spell grants the creature you touch the ability to understand any spoken language it hears. Moreover, when the target speaks, any creature that knows at least one language and can hear the target understands what it says.
It specifies that creatures hearing the target must speak at least one language, but does not specify that the target must. To me that seems like an exception that proves the rule, so I think Tongues can effectively be used like Speak With Animals, but on any one creature at a time (instead of all beasts).
Does that ruling seem consistent with other rules related to creatures that understand no languages?
This specifically came up because I'm planning a RAW-as-possible level 20 fight against the legendary tarrasque, and one of my players suggested they should cast tongues on it.
Related, but doesn't address Tongues spell: Do either Speak with Animals or Awakened Mind let me use Command on animals?