When using the MailChimp API 2.0 to subscribe a new user to a list you want to send opt-in emails. You want to use MailChimps auto translate feature (Lists -> Signup forms -> Translate it) so your subscribers receive localized emails.
In order to set the correct language for each user you probably already use the mc_language option within the merge_vars array as stated in the official documentation.
Problem: Emails are still localized using default language as set for the list in MailChimp.
Solution: You need to set the Accept-Language HTTP header for the API call.
Send localized MailChimp emails: set curl options for PHP API call
<?php $curlHandle = curl_init(); $curlHeaderOptions = array( 'Content-Type: application/json', 'Accept-Language: de', ); curl_setopt($curlHandle, CURLOPT_HTTPHEADER, $curlHeaderOptions); ?>