Having come across a few threads and supposedly relevant or relevantly effective websites, I still have not found the answer to my question. After trial and error and having read things here or there, it's about time the matter is solved once and for all.
It concerns changing the word „Chapter” (or whichever other equivalent in another language), while using XeLaTeX. First, I used, e.g.:
As it did not work, I came across some source implying one should perhaps consider babel and its presence, so I put that command up there just after the document command in the preamble. Well, it did not work. Moreover, in my case I am using polyglossia, so must I perhaps somehow consider it in this matter (perhaps somewhat similarly to the consideration of babel)?
What is the quick and clean solution?