I try to translate plugin to my language and found a small bug.
in modules/gallery/gallery.php at line 265.
change
'400' => _x( 'Large', 'Photo size.', 'fl-builder ')
to
'400' => _x( 'Large', 'Photo size.', 'fl-builder')
“Large” text can’t translate because of there is a space after fl-builder tag.