Covid RT-PCR test within 24 hours of arrival.

[goji]

Poor choice of words. Flak = Flugabwehrkanone (anti-aircraft gun). However, flak is now used in English as a general term (criticism, opposition) and the German 8.8 cm Flak during WWII was used efficiently against ground targets as well.

Had I been attempting to write in German, I'd have hopefully used a capital F, but would almost certainly have made far worse errors elsewhere.

To face the flak is a fairly common term used in English.
As is to "blitz" something, e.g. bliz all the weeds in the garden with weedkiller.

To criticize this would be rather like me criticizing Germans for misusing English words adopted into German, such as "Handy" (mobile phone).