When editing and saving a translation in the admin language pack interface, all array translations in the fie are replaced by empty strings. e.g. in ucp.php:

'FIELD_TOO_SHORT' => array(
1 => 'The field “%2$s” is too short, a minimum of %1$d character is required.',
2 => 'The field “%2$s” is too short, a minimum of %1$d characters is required.',
),

will become:

'FIELD_TOO_SHORT'',

This problem was not present in phpBB 3.0.x