I think it is not compatible.
UPDATE: As in https://www.phpbb.com/customise/db/official_tool/stk/
re. phpBB 3.1.x
The Support Toolkit is unsuitable for phpBB 3.1.x, at this time, and cannot be successfully installed nor run.
re. phpBB 3.1.x
The Support Toolkit is unsuitable for phpBB 3.1.x, at this time, and cannot be successfully installed nor run.
.Mo and .Po
one of the reasons for Drupal translation delay.