Based on what does translate appear only in some apps and some places it misses translated? I happen to open  a Chineese application and in that page all the info was Chineese so on top right corner there was a translate button On clicking translate it did translate the page but not the whole page.

Places like what's new section was still Chineese Why is it not translating the what's new section in some places it simply removed the text

I opened other Chinese app also and in that the translate  icon doesn't appear.

So based on what does the translate icon appear on some apps and also why does translate doesn't translate completely
Usually is up to the developers to translate their apps, so it means they had no time/didn't want to translate it. They probably chose to insert a translate button which uses something like Google translate to do the work, but gives approximate results.