Hi, there was a bug in the welcome email management with Polylang, it should be solved. Could you check once again? Some other issues have been solved on latest version, always related to multilanguage configurations, for example, the welcome email was not picked up in the correct language.
Let me know if the problem persists, dealing with multilanguage plugin is not always easy.
If you use custom forms, you can add the language to those forms with an hidden field named “nlang”. That value is stored with the subscribers’ data and should drive all messages (except when there are nugs :-).
Thank you for your collaboration!