English writing on beach?

I'm not Korean, not do I personally know any Koreans, but my guess is:
It's easier to write and see at a distance than any of the Korean alphabets (like the other major Asian languages, Korean can be written in several different alphabets);
Most Koreans know at least basic English words like "HELP";
English is considered the "international language", so is the natural choice for a distress call.

There're historical precedents for a foreign "international" language being used for distress calls. Perhaps the best known is the "Mayday Mayday Mayday" call used by ships and airplanes in distress, which is an English language corruption of the French m'adair, literally "help me", or the older Morse code "SOS", from the English "Save Our Ship", which was used by mariners regardless of their native language.

The reason I got from the movie was, he initially writes "HELP" in English because English is the international lingua franca. Then he decides he wants to stay so he changes it to "HELLO" (easier to simply change help to hello than write something else, I guess).

Then he gets the first message in English and carries on corresponding in English as he assumes, as does the girl, that English is the only language they both understand.

"Maybe there is a beast... Maybe it's only us."