Another issue in terms of choice of language to contribute in could relate to their
motivation to add the content and presumed audience for the content. A multi-lingual
person might decide to write about (say) magnetism in English (or other widely spoken
language) in the belief that magnetism is of worldwide interest, but might choose to write
about a local folk story in a more local language in the belief that it is likely to be
of interest only to local people.
Also given that there are different policies on different Wikipedias, it may be that a
topic might not pass notability on English Wikipedia but be entirely acceptable on another
Wikipedia.
Also, my observation of English Wikipedia is that regular contributors tend to divide into
article-starters (a smaller group) and article-expanders (a much larger group). If there
are cultural reasons (or Wikipedia policy reasons) why people fluent in one language are
less likely to be article starters, this may limit the range of topics for the
article-expanders to work on and hence the growth of the encyclopedia overall. There may
also be cultural reasons why certain types of article are not started in some Wikipedias,
e.g. popular culture articles (e.g. Pokemon characters) might not be seen as
"encyclopedic" in some cultures.
As to the specific difference between Polish Wikipedia and South Korean Wikipedia, I would
observe that South Korea is a nation obsessed with computer gaming both for personal
leisure through to professional sport, and it is a very time-consuming passion.
https://en.wikipedia.org/wiki/Video_gaming_in_South_Korea
So maybe gaming takes away the time from those who might otherwise contribute to
Wikipedia.
Kerry