Most of the texts in Resrv are read from the addon's language file. This page explains how to change them or add new translations.
If you didn't publish them during installation, you first need to publish the language files:
php artisan vendor:publish --tag=resrv-languageYou should now see the language file in lang/vendor/statamic-resrv/en/frontend.php.
You can edit this file in order the change the wording used in most areas of Resrv's frontend.
When using multisite, you can simply create a new folder with the handle of your other sites in lang/vendor/statamic-resrv/. Then, simply copy frontend.php to the new folder and translate the strings.