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 locale_en_US.properties 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 locale_en_US.properties and rename the en_US part of it to something else. Then inside mcmmo.properties 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 messages_en_US.properties 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.