I was having translations issues too, I’m using WPML for a multilingual site and all the strings translated in the .po were displayed in the main website’s language.
Then I found this post and I moved the function
load_plugin_textdomain( 'piereg', false, dirname(plugin_basename(__FILE__)) . '/lang/');
in pie-register.php
from line 83 to line 376 (inside the pie_main()
function) and now the translations are working as expected.
Maybe you can consider this change in the next update.
I’m also trying to translate the registration form in each language but I’m afraid that it won’t be possible 🙁