The simplest approach is to make individual copies of all notification templates (order confirmations, invoices, quotes, approvals, etc.) of all supported languages - like OrderConfirmationEN, OrderConfirmationES, etc. Then, send them the version that is specified by the customer locale or group metadata (such as their Preferred Language custom field in the B2B Customer Group settings or a custom customer attribute).
When you are working with the native email templates with BigCommerce, you can localize them yourself via Advanced Settings - Email Templates, and then integrate with an ESP such as Klaviyo, SendGrid, or Mailchimp, and to implement multi-language logic in a more flexible way. Those tools enable you to route notifications using the language tag or storefront URL of the customer. In the case of B2B portal messages or quotes, you can also use GraphQL or REST APIs to send localized messages out of the box - e.g. via an intermediary service or workflow tool such as Make (Integromat) or Zapier.