I can’t drink anymore.
Mo nomemasen.

One more and I’ll be under the table.
Ato ippai nondara taoremasu.

I’m feeling drunk.
Yoimashita.

I’m wasted!
Yopparatta!

I want to throw up.
Hakitai.

Where’s the toilet?
Toire wa doko dess-ka?

I’m going to pass out.
Watashi wa yoi-tsuburemashitai.

I’m never, ever drinking again.
Mo nido to nomimasen.

I really, really love you.
Honto ni anata ga suki dess.

I’m so hungover.
Futsuka-yoi dess.

What did I do last night?
Watashi wa yube nani o shimashita ka?

You are friends when you are naked.
Hadakano tsukiai.

And some other useful phrases from different languages…

Go to the devil.
Duten dracu.
(This is Romanian…submitted by my roomie Simona.)

You’re the fat piece of meat on the devil’s table.
Est carne grasa pe masa lu dracu.
(Also Romanian.)

I love meat!
Yo ubesc carne!
(Also Romanian.)

These pretzels are making me thirsty!
Diese pretzeln machen mich durstich!
(German. Submitted by my buddy Cornelia.)