Hello!
When I searched indonesian translation for phpBB 3.1, I found nothing. Therefore I took a standard english laguage files and replace all lang variables which I could find in indonesian langfiles for phpBB 3.0.
Result of my work have been placed on github here:
https://github.com/FladeX/phpbb_id. Everyone can help with futher translation: just make a pull-request to my repository.