You'd have to use APKTOOL to decompile the APK's that need translating. Then go to RES\VALUES\strings.xml and edit away. Afterwards, recompile. Some may need for you to take the framework-res.apk and incorporate it (apktool -i framework-res.apk). In fact, run that first and then decompile.
There are other files that may need to be edited, but this will get you a ROM that's 95% translated.
Just a word of warning, some files are not that easy to recompile.
What needs to be edited: