Hi there @Leo Pure Electric,
Thanks for posting to the Community! You provided a great question and a good scenario that other Community users have also discussed.
So you are right in the sense that there is no way to distinguish different languages from the same country. Right now, the Klaviyo method to doing this was for customers who were already based in different countries with different languages. At this time, we do not have the capability to pair the language denoted in the browser URL with the flow content.
We're not able to account for different languages automatically unless you are already collecting subscribers and added a "Language" property. Shopify's Checkout Started metric does not pass website language data so we can't use that dynamically. We recommend using a signup form to collect a Language preference. You can then use a Flow Filter or Conditional Splits within any Flow to filter for this language property. This is outlined here in more detail: How to Customize Content Based on Language.
You could then create flows filter for segmented customers who either selected French/Dutch and then have different templates built for those languages. I’d also be more than happy to have this looked at as a product request to see if we can get this possible as a feature in the future for the Klaviyo product.
Feel free to reach out if you had further questions!
Alex