Hello,
Let use the ordered list for the questions if you post more than one question in the same topic.
1/ Vendor registration. We need FTP access to check the issue in more detail.
2/ Use Volume pricing table option https://gyazo.com/853c24850f23fa5db85ffec5bbc33826
3/ Here you can find the list of multilanguage plugins https://colorlib.com/wp/wordpress-multilingual-plugins/ Our theme supports them. To add switcher you need to check the plugin documentation and use the language switcher shortcode or widget in the header.
WPML plugin is most popular.
4/ Email Woocommerce notifications not working after ordering – describe issue in more detail, please. Don’t users get notifications or admin?
Regards