Transfer language to Maileon
In order to make the language transfer work, you will need the Shopify Flow App, which is available from the Shopify App Store.
You need to install it and then create a new field for Customers in the settings under "metafields".
Here, make sure that the Namespace and key is "customer.lang".
This is the name of the variable, and it appears in the API with this name, so please make sure you use this name.
Also you have to check the box: "Expose this metafield to Storefront API reuqests."
If you save it, the metafield will be created for all customers.
In order to sync the language to Maileon, you have to set up an automation with the Shopify Flow App mentioned above (this is similar to the Marketing Automation in Maileon).
The following should be built here:
Customer created --> check if he/she is a newsletter subscriber --> if so, add the language that is already in Shopify to the metafield.
If this flow is created the language will be synced to Maileon.