I just sent this as a support ticket but I guess it’s more of a bug report/feature request, so I’ll post it here as well:
WP 4.7 introduced user-specific language settings (https://make.wordpress.org/core/2016/11/07/user-admin-languages-and-locale-switching-in-4-7/) unfortunately these are not picked up by ACF.
Since this also affects front-end ajax calls for logged in users I don’t know how much of a problem this will cause.
I suppose it should be something along the lines of
$locale = (function_exists('get_user_locale') && ! defined('ADMIN_AJAX')) ? get_user_locale() : get_locale();
Thanks,
Alex
Since this is now a standard feature of WP, I suspect that the developer will add this to his to do list. In any event, submitting a support ticket was the correct thing to do.