Planescape languages

[Heatwizard]

I'll be playing in a Planescape campaign, a setting I'm not terribly familiar with. I've got three bonus languages and no idea what will end up useful. Are there any specific languages that see a lot of use in Sigil?

[Tvtyrant]

Infernal, Abyssal, and Draconic are always good. Maybe Sylvan.

[Luckmann]

Celestial, Infernal, Abyssal.
Those would be my picks, if available, for Planescape.

[Zaydos]

Infernal would be my first pick, in 2e Planescape it was also known as The (Lower) Planar Trade Tongue and hinted at as the roots of common.

Celestial and Abyssal would be my next two followed by the 4 Elemental Languages.

I would not advise Draconic; dragons tended to be rare on the planes. Although they did make one for most every outer plane in 3.5.

[Eldan]

Ask your DM if he goes by the common variant rule that Planar Trade is the same language as common. Because technically, Planescape did not do that.

Apart from that, it depends what your DM uses. I've seen a lot of DMs use the alignment trade languages: Fiendish for the lower planes (also called Lower Planar Trade), Celestial for the upper planes (where technically, all three outsider species speak their own), and equivalent languages for lawful and chaotic.

Any common outsider language is never wrong either.

Apart from that: guess where the adventure might go. On the chaotic half, Sylvan is never really wrong. Picking up Giant for Ysgard isn't a bad idea. If you head to the elemental planes, pick up a few of those languages. The City of Brass is a common trade place, pick up Ignan.