For example, a sprite knows Elvish and Sylvan. Assuming the owner does not speak these languages themselves, would the sprite be able to translate for the owner? What about the pseudodragon which understands Draconic and has the "Limited Telepathy" feature?
Any creature can act as a translator
Under RAW, languages are binary: either a creature doesn’t know a word or they are completely fluent and their vocabulary is 100% complete. If they know two languages, there is no reason that they cannot act as a perfect translator for two other creatures that know only one of those languages.
It doesn’t matter if the creature is a familiar or not.