Most animals don't speak any languages…
If you look at the "Languages" entry in their Monster Manual entries, no languages are listed. For example, the entry for the raven says (MM, p. 335):
This is important, because being able to understand at least one language is often used as a measure of communication ability. For example, the description of the thri-kreen racial trait Thri-Kreen Telepathy states:
[...] A contacted creature doesn't need to share a language with you to understand your thoughts, but it must be able to understand at least one language. [...]
As a result, thri-kreen cannot use this feature to communicate with ravens: the ravens simply don't have the level of sapience required. Whatever method these birds use to communicate with each other, it is not a language in D&D terms (or in real-world terms, for that matter!).
This also means, if you want to understand what a raven is trying to communicate, you'll need to use a spell that doesn't specify languages. Speak with animals allows you to "comprehend and verbally communicate with beasts" without requiring that they speak or understand any language; this is presumably the expected way to communicate with animals like this.
…though some do.
If you look at the stat block for the giant owl (MM, p. 327), for example, you see that it speaks the Giant Owl language. Since this is an actual language, comprehend languages will let you understand it, and thri-kreen will be able to speak to giant owls telepathically.
So while most beasts don't speak any languages, you'll need to check the stat block for a particular one to be sure. A small handful are intelligent enough to mechanically have full languages of their own.