I know that it's not that well explained, but I was searching for this a long time ago and didn't really find anything, this is just from experience, so don't expect too much of it.
It's really not that hard:
1. Rename the .mtz to .zip
2. Open it with whatever you open your zips with.
3. Anything you can't open, just open it with winrar or whatever.
4. Search through everything and open manifest.xml (I think... most of the time the rest is just ****)
5. In that manifest.xml, you just search for anything that looks chinese and use google translate to translate it. ( if the translation sucks, just make a normal sentance of it)
6. Save the changes and just try it out. (be sure to rename it to .xml first)
Hope it was usefull.
(I know my English isn't always that good, but please ignore it)
PS: (Some things might not be up to date anymore, because I'm not really on MIUI anymore)