This thread is resolved. Here is a description of the problem and solution.
Problem: Can't translate custom tab from Flatsome Child theme Solution: I think that the best course in your case will be by using Multilingual Tools Plugin. I have installed this plugin on your site: https://wpml.org/documentation/related-projects/wpml-compatibility-test-tools-plugin/
And now you can generate your custom XML files. Do not save them into the theme directory, but on your computer instead and you can then take the XML generated files and add these into WPML -> Settings -> Custom XML Configuration.
Please have a look at these images - I have done it for the theme_mods_flatsome-child domain, specifically for the tab you showed in your image. These strings are now available within string translation. You do not have to generate these strings for all the options, but only for those you will use and you can find them by their content.
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