Tom I tested this with Korean Hangul I copied from a news site.
I pasted it into a plain text Textedit document and stretched/shrank the window to check it has no line breaks then copy pasted that plain text to Pages 6.3.1.
No sign of any line breaks and word breaks are fine.
Then reduced the margins and the text flowed normally in Pages.
I can only assume the O.P. has done something else or has text with non-break word spaces.
There appears to be no problem in Pages 6.3.1 Sierra.
I started setting up a drive to run High Sierra this morning but backed out when it started looking a little dicey and looked like it might screw up what I had on the drive. I might get another brand new hard drive just for High Sierra.
Peter