I've rarely seen it referred to as "WW2", as opposed to "WWII", but apparently it's been in relatively rare but consistent use in English [1] for a long time.
Tweaking the query reveals that it's relatively more common in British English than English overall, although "WWII" is more common there overall as well.
> I've rarely seen it referred to as "WW2", as opposed to "WWII",
I'm an older Brit and had to think about this one but it's usually been WW2 as far as I can remember. I think the WWII is perhaps more recent or American usage.
I've just had a look and The Commonwealth War Graves Commission refers to it as WW2, or in words as "World War Two" / "Second World War"
Tweaking the query reveals that it's relatively more common in British English than English overall, although "WWII" is more common there overall as well.
1. https://books.google.com/ngrams/graph?content=WWII%2CWW2%2CW...
2. https://books.google.com/ngrams/graph?content=WWII%2CWW2%2CW...