On the Booking Listing page exist ability to change language (locale) of emails, that will send to visitor during approving or declining of the booking. But how to detect language/locale of webpage, from where visitor submit the booking.
Here is instruction how to do this.
Please open the Booking > Settings > Fields page and add to the booking form this shortcode and JavaScript code:

<div style="display:none;"> Active Language: [text active_language class:wpbc_lang_field]</div>
<script type="text/javascript">
 jQuery(document).ready(function(){
    jQuery('.wpbc_lang_field').val( wpbc_active_locale );
 });
</script>

Its will add to your booking form new field [active_language] and JavaScript for auto-inserting active locale into this field. Please note, this field is not visible for visitors, because of this CSS: style="display:none;"

Also you will be need to add to the "Content of booking fields data" form (its at the bottom of Booking > Settings > Fields page) this shortcode:

<strong>Active Language</strong>:<span class="fieldvalue">[active_language]</span>

For showing this field at the Booking Listing page.