Not finding a solution, I approached it differently. I created a new endpoint, /mon-compte/vouchers/download. I pass the current language in the arguments of the download button on my /en/mon-compte/voucher page. I handle the click with JS, then send it to the new download endpoint, which is managed in store default language. I retrieve the language passed in the arguments, and this way, I return the corresponding PDF without worrying about the translation plugin