Hello,
I have been developing a website using this starter:
I have two languages in my prismic dashboard. Everything is setup and working fine when switching languages, it loads fast and properly the right content.
I would like to have browser detect language and once a user opens the URL to display the content in the language of the browser.
If I understand your question well, you can use navigator.language in your js and get the client browser's language in your code. I found this document which can be helpful for you.
Please let me know if that does not answer your question well. I would like to help further.