The guy shooting is Robert Hirt, the (lead) designer. There is a translation by HKpro member Lenkers (below) that is accurate:

0:00​ - 0:05​ Hirt: "G36 No. 205 fire to destruction."

0:09​ - 0:09​ Backround: "Slow down Robert, everybody wants to attend."

0:10​ - 0:25​ Discussion on how to hold the rifle.

0:32​ "Open fire, or?!"

0:33​ "Attention!" (brrrrrrrrrrrrrrrrrrrrr commences)

0:40​ - 0:50​ Backround: "The ejection is flawless" (Note the guy in the backround, totally exited about the trajectory of the empty shells)

1:29​ "The fourth one? Yes"

1:45​ "Be careful, the gas block is getting red."

2:00​ "600, yes? Yes!"

2:13​ "Now its getting hot. 700? Yes!"

2:33​ "We've done it, now it burns."

2:35​ "I owe you a beer (?)"

2:37​ Hirt: "The bolt hold open doesn't work anymore."

2:41​ Hirt: "Let's shoot some more?" (Literally: Shall we throw out some more?)

2:44​ "Better take care!"

2:55​ "Throw it away, stop it!"

2:59​ Hirt: "Do you have one more?"

3:03​ -3:08​ "How many? 800! But be careful!"

3:08​ Hirt: "One more!"

3:17​ "Throw it away, now! Throw it away, NOW!"

3:24​ "Well done!"

3:27​ "Lick my ass"

3:29​ Hirt: "205 after 900 rounds sustained fire one magazine 30 rounds. Bolt hold open works, wicked! It works! G36 205 after 900 rounds one magazine 30 rounds runs without complaints."

3:53​ Hirt: "GEIL!"