Actually MIUI is stable enough right now...
The version we get in India out of the box is good enough too (just a few minor bugs here and there, nothing to worry unless you have OCD
)
Yes, a reset/wipe will remove your Gapps... That's because these apps dont come as system apps on Xiaomi phones...
And to get rid of the Chinese language in Themes app just clear the data of Themes app from settings (Its not a permanent solution but will atleast make it usable for now) If you get the same problem again just repeat the said work-around..
Have fun with your amazing Mi3...