Hi Martin,
Thank you for your question. Real Cookie Banner has support for multilingual websites with WPML and Polylang, as you can see in the plugin description on www.remarpro.com or on our website.
Your specific question about working with the WordPress Customizer has already been asked by another customer this week, and we will write a detailed article about it in the next few days. I will post in this thread when the article is available.
In short, an answer to your question: WPML and Polylang have no visible integration with the WordPress Customizer. So you can’t switch between languages in the WordPress Customizer to change the settings per language there. However, the multilingual plugins offer so-called string translations. You can learn more about this feature in the documentation of your multilingual plugin:
WPML String Translation: https://wpml.org/documentation/getting-started-guide/string-translation/
Polylang String Translation: https://polylang.pro/doc/strings-translation/
In String Translations you can find any text you can change for Real Cookie Banner in WordPress Custimizer. Just use the string translation tool of your multilingual tool to translate the string to the language of your website.
I hope this explanation helped for the moment!
Best regards,
Jan