I could find the theme into our official WPML-compatible themes directory (https://wpml.org/theme/prophoto/) but it was tested with version 5 so that was back in 2015.
From what I could find here on their site (hidden link), they are saying:
"However, our developers are working on adding support for the WPML plugin in the future so you can use the WPML String Translation add-on to provide translations for the text you’ve used within your ProPhoto layouts.
For an estimated date for WPML support, please refer to our ProPhoto 7 development roadmap."
If you click on the development roadmap link on their site (hidden link), you will see that they say the theme it's going to be compatible with WPML on 25 Feb - this month.
So it seems that the theme might not be fully compatible with WPML at this moment but it's going to be in a few weeks.