We find two contradictory statements in the given data.
First, the British Guy stated that he was in the cold storage room selecting meat for lunch.
Secondly, the French told that he was sleeping after the Night Shift.
These two statements force us to think that the culprit may be one of those.
However, the actual culprit is the Sri Lankan, who said that he was correcting the flag. But looking at the Japanese Flag, we find that it is the same even if turned upside down. So, the Sri Lankan is the culprit.