First of all use an archive program like WinRar or 7zip to open up mcMMO.jar and navigate to com\gmail\nossr50\locale\ inside the Jar file. Here you will see a file called this is the localization file for en_US and will be the template for other languages. You can either modify all strings in this file or copy it and start making a new file for localization.

If you want to make a file for another language you would copy and rename the en_US part of it to something else. Then inside you will change the localization file setting from en_US to whatever the new files name is. You would have to add your new localization file to the same location in mcMMO.jar that is in. For example, If you were translating mcMMO to Mexican Spanish you would name it es_MX. If you want to know what the correct name for your localization would be you can check the ISO 639-1 Table. I'll probably change these files to be located inside your plugins folder in future versions.

Anyways theres a few things you should know about the localization files, they use special words to insert color codes, for example [[GREEN]] will insert the green color code. These are based on the ChatColor bukkit uses found in this source file. So if you want to insert RED used in bukkit, write [[RED]] in the localization file.

If you fully translate mcMMO into another language contact Nossr50 to get it included into mcMMO.