@Jakub - For a quick fix, you can just create exclusionary Flow Filter rule from one Flow to the other. For example, if they were going into Language A Flow, you would exclude them if they’ve already in the List of Language B (or, received emails from Language B Flow). And vice versa.
A better approach, similar to the one I propose using customer properties for your OfferA vs OfferB works here too. So you can have both Language specific Signup Forms submit to the same List - let’s call this your “Main List.”
Then, in the Signup Form, you have two hidden custom properties, one for each Language
- LanguageA = True
- LanguageB = True
Then you can create a Segment of Language A or Language B. You can then just have a Conditional Split in a single Welcome Series. Since either form sends users to the same “Main List” - they can’t sign up more than once and go through the Flow since List Triggered Flows can only be received once.
Or if you prefer to create two separate Welcome Series with a Flow Filter for either Language A or Language B, then add the exclusionary rules from each other like I mentioned at the very top.
Using Custom Properties gives you a bit more flexibilities for future Flows and even in your Templates and Custom Consent pages down the line.