Before starting to translate, you will need to download and build the game sources: [BuildHowTo]

Adding a new language support to Newton Adventure need two steps:

• create a messages_${language}_${country}.properties file in the game/src/main/resources directory. Check messages.properties (default, english) and messages_fr_FR.properties file (french) for example. Note that you cannot write these file with utf-8 encoding. They must to be encoded as ISO-8859-1.
And yes java api choice sucks.
• create image button for each file following the pattern bt-*__${language}_${country}.png. They must be 256x256 32 bits RGBA png. You can create them using the button.svg at the end of this page as a template and the export function of Inkscape. Again check the french file as examples.