I was excited that you finally added internationalization with v1.2.9 - and that is really important! So, a big "Thank you!" :-)
However, there are some errors/typos in the latest version which I found out with testing:
In file "ninja_forms.php" you have:
load_plugin_textdomain( 'ninja-forms', false, $plugin_dir );
it should go this way, otherwise translations will not load:
load_plugin_textdomain( 'ninja-forms', false, $lang_dir );
Then in both files "includes/body_settings.php" and "includes/body_subs.php" you have typo errors - you used this (9 times each file!):
but it should be this way:
--> This error leads to fatal errors on the forms and settings page!
Otherwise you have a lot of gettext strings without textdomain - you should always only use "ninja-forms" as this is the one you've defined and are loading!
__( 'Your string', 'ninja-forms' ); _e( 'Your string', 'ninja-forms' );
Also please use single quotes for all in all gettext strings - it's the recommended way!
Thank you very much!
-Dave from Germany
P.S. Currently translating your plugin into German :)