That’s a great question and I can definitely understand the value of being able to add that tag automatically and filter customers by language.
Auto-tagging is not a feature currently supported from within the admin and would require a custom solution to setup. There are a range of apps available in the app store that can provide custom tagging options, like this one: Auto Tags ‑ All in one tagging.
This could also be done through a custom app built for your store.
I would recommend contacting Klaviyo first though before going with those options. They may have a reliable solution they can recommend or maybe even offer it within their app as part of the customer account creation process.
We have done this for a client. What you need to do is in Klaviyo create 2 signup forms, 1 for English and 1 for French. Then show the form based on the language url, this way your customers go in their own list english or french.
If you are having a Shopify Plus then you can use Shopify Flow to achieve this.
You can create the following flow
When the customer is created > Check if the customerLocale includes en > Tag the customer “English”
(If the customer places an order/ customer subscribes to the newsletter from the English URL, The value for “customerLocale” will be “en-CA” and for French, it will be “fr-CA”)
Later in Klavio, you will be able to create a list with a customer tag filter.
If you have a Shopify Plus then you can use Shopify Flow to achieve this.
You can create the following flow
When the customer is created > Check if the customerLocale includes en > Tag the customer “English”
(If the customer places an order/ customer subscribes to the newsletter from the English URL, The value for “customerLocale” will be “en-CA” and for French, it will be “fr-CA”)
Later in Klavio, you will be able to create a list with a customer tag filter.
I have posted a solution to this thread, I was able to do this with Shopify flow to add customer tags and later create a list with a tag filter in Klavio.
Can you share a screenshot of your flow? I just tried it again and it works for me.
Try checking Recent runs under your flow after creating a test customer. Make sure you have the correct condition written from the above example.