Thank you for contacting WPML Support. To translate text from Customizer/Theme options, here's a simple guide:
1. Go to WPML >> String Translation page.
2. Scroll down and click on the Translate texts in admin screens » link at the bottom of the page.
3. Search for the your required string.
4. Select the string.
5. Click on the Add to String Translation button.
6. Then click on "« Return to String Translation" link.
7. Add the translation from WPML >> String Translation page.