Lingua Franca

Choosing bonus languages in standard D&D games is the worst. Either your choice matters a lot ("Oh, we're playing Keep on the Borderlands? Great! I chose Goblin as one of my starting languages.") or it matters not at all ("I speak Mermaid! What do you mean we're in the center of the desert for the whole campaign?"). 

I've read some good advice on how to make the languages in your campaign setting more meaningful, but one of my favorite techniques is to make the language, itself, have some sizzle on its own. 

(This core of this idea was borrowed (like so many things) from the GLoG. I think I first riffed on these for my GLoG race templates, here. But they seemed worthwhile to pull into their own post.)

Common

Common can be understood by everyone. Literally everyone. It's a magical language that forces its meaning into your brain. If you don't speak Common, this is very uncomfortable.

Elvish

Elvish is comprised of psychic syllables. Speakers are totally silent, but if you speak Elvish and are looking at the speaker in the eyes, you can hear them. You cannot tell a lie in Elvish.

Dark Elvish is a corruption of the Elvish language. It's also totally psychic and silent, but you can actually lie in it (if you're proficient).

Halfling

Possibly related to Elvish, Halfling is another silent language. Illusory word bubbles appear above the halfling's head. You don't have to look your subject in the eye--when yelled, the word bubbles are visible at a distance. The word bubbles look somewhat ridiculous, like Mr. Saturn's talk.

Dwarvish

Dwarvish is untranslatable. Spells like Comprehend Languages/Tongues automatically fail. If you don't speak Dwarvish, it's an untranslatable code.

Gnomish

Birds can understand Gnomish. They don't really care, but they understand it. 

Draconic

Draconic is hot. Speaking Draconic literally warms you up. In even mild climates, speaking Draconic is uncomfortable for humans since it raises their body temperature so much.

You cannot write Draconic on paper - it catches on fire. Only stone can bear the heat of its glyphs.

Goblin

Goblin hangs in the air after it's spoken like an invisible stinky cloud. Goblin houses get stained and greasy from the continual stream of their chatter. Goblins, dogs, and anything else with a great sense of smell can detect it for days after it's spoken. (Credit to Brent at Glassbird Games for this one.)