A raven familiar already speaks and understands at least 1 language. If the fiendish template is applied to it, it still already meets the requirements of speaking and understanding at least 1 language. The fiendish template doesn't say, "Creatures with the fiendish template gain understanding of an additional spoken language." It only implies indirectly that if the base creature didn't already know how to speak a language, it would gain the capability now to understand (but not necessarily speak) one language, presumably common, since nothing else is noted.
I think it's actually better to start with a fiendish raven and then make it a familiar, rather than stat with a familiar and make it fiendish, since according to 3e metaphysics a familiar is an already existing creature rather than a created or summoned being. That said, I think you are probably right that the text doesn't indicate it would gain a new language. It kind of bugs me though, since the fiendish template is a rather underwhelming addition to a familiar, and giving it an extra language would at least be something.
However, I'm not sure that that outcome - though irrelevant to the raven familiar - is actually intended. I'm not sure that I read "Abilities: Same as the base creature, but Intelligence is at least 3" as clearly indicating it gains any unstated abilities, even those that normally come with having an intelligence increase - such as language ability or increased skill points. "Same as the base creature..." could be read as an exemption to the normal rules that enforces that only the intelligence increases and all secondary abilities (skills for example) remain the same.
From what I can tell under the general template rules, since a fiendish creature is a magical beast a fiendish creature that began as an animal gains the traits of magical beasts, but keeps the features of an animal. Animals have a trait that specifies they have an Intelligence of only 1 or 2, while magical beasts (like most creatures) have no such trait. So it seems like magical beast is the standard to be working from, and if that is so, a understanding of at least one language is assumed for a magical beast with an Intelligence of 3+.
Since magical beasts do not, however, have a
minimum Intelligence of 3, I think the note that a fiendish creature has a minimum Int is intended to support that attribution of (im)morality to the fiendish creature--ie, to allow an evil alignment in the same way a fiend would have one.
Thanks for your thoughts. I was hoping someone would find some sort of rule I missed that would say, "yes, a fiendish raven familiar speaks 2 languages!", but I guess I'll just have to talk with my friend and see if he will allow the familiar to have both Abyssal and Common for story reasons.