Not sure if this answer qualifies according to subreddit rules, but in short:

Japanese-Russian border conflicts never materialized into full-scale war. A couple of divisions on each side shot at each other a few times, then Japanese got get their ass handed to them after the disastrous battle of Khalkhin Gol and decided not to poke the bear any further. The Russians were focusing on Hitler and Europe at the time, and had no interest in fighting Japan, since they had nothing to gain.

Japanese invasion of China was a localised conflict, since no other powers were involved. To Europeans of the time, "China" wasn't really "people" - it was a backwards third world country (in the colloquial definition of "not important"), too big to be conquered, and only worth caring about for the export market and cheap labour.

Hitler's annexation of Czechoslovakia happened in Europe, which sidelines your point about Eurocentrism. But it was a local conflict that didn't have any other consequences beyond some diplomatic repercussions and Chamberlain hiding his head in sand thinking it'll all go away if you keep Hitler happy.

Which brings me to my main point. Hitler's invasion of Poland is what started the domino effect that started the actual world war. I.e. England and France declared war on Hitler pretty much immediately, while the USSR entered the war ostensibly on the side of Hitler. At this point, 4 out of world's 5 major powers were at war.

It's not that the other conflicts were insignificant in their own right, it's just the major powers didn't get involved in them.