
Joyd |
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"Demonic" isn't a language. Demons speak Abyssal as a main language - and often Celestial and Draconic, but rarely Common. The rare demon that relies on language-dependant effects or is a charasimatic-type demon tends to have Tongues available. A few demons know Infernal, but it's a rarity.
Devils speak Infernal as a main language, and often Celestial, Draconic and Common, though not all speak Common. Very few speak Abyssal.
As far as which is better, you're more likely to have a language in common with Devils by default, since way more of them speak Common. On the other hand, Devils are generally talkier than Demons by default fluff, so you may value the ability to understand what they're saying, speak to them in their own language, or read their writing. (Devils are way more likely to write things down.)
Strangely, if you want to speak to Devils AND Demons with a single language choice, Draconic is probably your best pick. Nearly every devil or demon speaks Draconic.
Daemons, on the other hand, tend to speak both Abyssal and Infernal, but not Draconic, so if you want to talk to them, you're back to those. (They also rarely speak Common.) Divs are similar, but additionally speak Celestial. Oni and Rakshasa speak common just fine. Rounding out the families of random evil stuff, Qlippoths speak Abyssal only, but they all have telepathy.

Alitan |
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OK, so, in my opinion, there's no use talking to demons. They lie, they're treacherous bastards, join the Blood War and just kill them.
Devils, on the other hand, while hair-splitting, fine-point-arguing... well, Hells, they're kinda like people on the messageboards, come to think of it. Anyway. If a devil gives it's word, nine times out of ten it'll keep it. Of course, six times out of ten, it didn't promise what you THOUGHT it promised, but that's your fault for failing to ask the right question and specify, specify, specify. Written contracts are better than verbal ones.
But learning Infernal can be very rewarding, with the properly cautious approach.
And demons? Yeah, just kill 'em. Kill 'em hard, kill 'em lots. Or catch them with Trap the Soul and use them to bargain with devils!