I am a user of MS as well and have three comments:
First, merits aside for your suggestion for the moment, in the US English units are still quite common in science and engineering disciplines. (Long story).
Second, as to the suggestion itself, I am a bit puzzled. I do not use the built-in MS units much, I have my own routine that allows me to move between any unit system. But as best I can tell, there are no default units. MS will calculate the results in whatever units you enter and display the result in the same units. I know of no default behavior that automatically converts them to English. Perhaps I am missing something here.
It will use the whatever set it sees first. For example:
Third, I have an iPad Air 2 and have not seen many crashes of late. It may simply be that we use the app differently and indeed there is a memory handling issue, but it MIGHT also be due to a bad app update. This rarely happens, but from time to time it does. Could be a wifi hiccup, could be some leftover junk that was left behind during some past update.
Anyway, I have developed the habit of deleting and then re-downloading and re-installing apps that are behaving badly. This has resolved issues more often than not. You might give that a try. Make sure you save off any valuable documents first. If they are in iCloud they will be restored automatically except for the compiled scripts. You will need to recompile the compiled scripts. Any non-iCloud files will need to be saved manually of course.