This thread is resolved. Here is a description of the problem and solution.
Problem: Cannot translate "Additional global tab" title & content in the Flatsome theme.
Solution: 1. Go to WPML -> Strings Translation and click the "Translate texts in admin screens »" link you'll find at the bottom of the page.
2. In this page you'll find a large list of admin options strings that are registered on your site. Here you should find the options that you're trying to translate. Maybe they're initially hidden inside an options group (themes and plugins are often grouping their options in a group). I'd suggest you to search for "Envoi". If you don't find it, try searching for 'Flatsome'. If you see a group of strings (a blue link with '+' icon at the left) with matching name, click on it and related strings will appear.
3. If you find the strings you want to translate, select them (checking their checkboxes) and click 'Apply' button (at the bottom of the page).
4. Go to WPML -> Strings Translation and search for these added strings. You should find them there and be able to translate them (make sure you mark them as 'complete').
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