The guide can help: http://miuiandroid.com/community/threads/miui-developers-translation-guide-v1-0-much-to-add.2492/ You can also https://github.com/miuiandroid/MIUI-ROM-V4.0--Ice-Cream-Sandwich- clone this repo. This holds the translations. Then make a folder called values-xx (where xx is the 2 letter language code of your language). Then translate those XML files.
That will be covered in the translation guide I posted above. You have to decompile the APKs, add the new XMLs, and recompile back into APK form. Then re-put into ZIP and re-sign it.
I install the rom and the Settings apk is not installed... I tried to install the Settings with "Root Explorer" from the system>app but it says "can`t install this app" (Sorry about my English...)