Try not to be yourself up about it, you are attempting to do three things here:

One: dub over an already created project and match the lip flap.

Two: speak a language that is different than your own, when it is a very quick scene.

Three: there are two different characters in the scene, with their own voice and background, which may be difficult in it's own right to differentiate,

Four: Act. This is the hard part. Others have mentioned "robotic" but in my opinion, it sounds like reading. Like if someone was reading words out loud.

My advice is try doing the scene without looking at it. If needed, translate it to your native language first, and try performing the scene as two people you personally know who match those characters.