Hi
In fact you must create the wpml-config.xml as it is not provided with the theme.
You need to find the name options of your theme and complete the file below. Search these names options either in your DB in the wp_options or in the theme php files.
Before I suggest your read this doc: https://polylang.wordpress.com/documentation/documentation-for-developers/the-wpml-language-configuration-file/
<wpml-config>
<admin-texts>
<key name="theme_mods_full-frame">
<key name="fullframe_theme_options">
<key name="promotion_headline" />
<key name="promotion_subheadline" />
<key name="promotion_headline_button" />
<key name="promotion_headline_url" />
<key name="featured_content_headline" />
<key name="featured_content_subheadline" />
<key name="featured_content_type" />
</key>
</key>
</admin-texts>
</wpml-config>