Probably, but not necessarily

While Leia reacts to Jabba directly, there isn't a scene where she is doing so verbally that Threepio is not translating.

From the transcript:

While

Leia is disguised as Boushh

We see:

Jabba continues speaking, as Threepio nervously translates. Boushh listens, studying the dangerous creatures around the room. He notices BOBA FETT standing near the door.

THREEPIO Oh. The illustrious Jabba bids you welcome and will gladly pay you the reward of twenty- five thousand.

BOUSHH (in Ubese subtitled) I want fifty thousand. No less.

Later, Leia may be reacting as much to Jabba's physical actions as verbal ones:

LEIA We have powerful friends. You're gonna regret this...

JABBA (in Huttese subtitled) I'm sure.

Inexorably her lovely face moves to within a few inches of Jabba's ugly blob of a head, and Leia turns away in disgust.

LEIA Ugh!

Which is pretty much the last bit of dialogue for her and Jabba. They don't seem to have much to discuss after this...

However, considering the following:

1. Leia was a Senator and diplomat and therefore probably knew quite a few languages. She apparently learned Ubese by this point, for instance.
2. The Hutts are a powerful business entity in the galaxy, so Alderaan has possibly done business with them.
3. Han certainly understands Huttese, so it isn't uncommon for Humans to learn.

It's not unreasonable to assume she understands the language. But from the movies at least, no confirmation that she does.

Probably, but not necessarily

While Leia reacts to Jabba directly, there isn't a scene where she is doing so verbally that Threepio is not translating.

From the transcript:

While

Leia is disguised as Boushh

We see:

Jabba continues speaking, as Threepio nervously translates. Boushh listens, studying the dangerous creatures around the room. He notices BOBA FETT standing near the door.

THREEPIO Oh. The illustrious Jabba bids you welcome and will gladly pay you the reward of twenty- five thousand.

BOUSHH (in Ubese subtitled) I want fifty thousand. No less.

Later, Leia may be reacting as much to Jabba's physical actions as verbal ones:

LEIA We have powerful friends. You're gonna regret this...

JABBA (in Huttese subtitled) I'm sure.

Inexorably her lovely face moves to within a few inches of Jabba's ugly blob of a head, and Leia turns away in disgust.

LEIA Ugh!

Which is pretty much the last bit of dialogue for her and Jabba. They don't seem to have much to discuss after this...

However, considering the following:

1. Leia was a Senator and diplomat and therefore probably knew quite a few languages. She apparently learned Ubese by this point, for instance.
2. The Hutts are a powerful business entity in the galaxy, so Alderaan has possibly done business with them.
3. Han certainly understands Huttese, so it isn't uncommon for Humans to learn.

It's not unreasonable to assume she understands the language. But from the movies at least, no confirmation that she does.