IMC, some languages have drawbacks.
Knowing Draconic gives you a +2 on Spellcraft checks, but a -2 to resist a dragon's fearful presence, and a -2 to resist any dragon's spell-like abilities.
Knowing Abyssal gives you a +2 to Intimidate checks (made in Abyssal), but a -2 on saves vs. any supernatural or spell-like ability of a demon.
Mortals cannot lie in Celestial.
Oaths made in Infernal are eternally binding, and if you try to circumvent the letter of the oath, it will be made manifest in a most horribly literal way. (Circumventing the spirit is fine.)
Secrets spoken in the Elemental tongues are said to be heard in the dreams of the princes of elemental evil. So be careful with them.
It's impossible to make an Intimidate check in Sylvan, but you get a +2 bonus on Bluff checks.
Cheers, -- N