Hello. I don’t know if your form submission is done for site members or anyone who visits your site. So my note below is based on having logged-in users.
My experience with multilingual and site membership (see https://forum.wixstudio.com/t/no-programmatic-way-to-get-or-set-a-members-language-so-emails-go-out-with-the-correct-language) is that language preference is set in the member record at time of registration. Thereafter, it can only be changed from the dashboard UI (there is no programmatic way to access or change it). This isn’t a problem for automation-triggered emails as Wix is sending these and using the member preference. However, as I noted in the linked post above, I am at a loss for triggered emails since I have no way of knowing a member’s preferred language. This is a shortcoming of the multilingual system. Either give me an API call to get the language (e.g. return it from getMember) and I can make the switch in code or make the triggered email system automatically switch the email language, as is done for automation emails.