How do I can the language settings to English once the different individuals type in their birthday?

If you want to make the form multi-lingual, you can do it as shown on this guide: https://www.jotform.com/help/298-How-to-Make-Your-Forms-Multilingual 

Unfortunately, it is not possible to change it based on a condition, so the user will have to select the language from a dropdown before filling the the form.