Harvest only supports a single invoice template, which means that any time you change the language of your invoice fields (from Invoices > Configure > Translations), that change will affect all of your invoices — past, present, and future. 

I have multiple businesses. Can I use Harvest to invoice from each of them?

It is not possible to invoice as different entities from the same Harvest account. If you need to send invoices from multiple businesses, you'll need a separate Harvest account for each business. 

Can I invoice in multiple languages?

While Harvest does not support invoices in multiple languages, a workaround here would be to enter both languages into the fields of your Invoices > Configure >Translations page.

With invoices set to those defaults, both languages will appear in every field. This will allow your clients to read whichever is most comfortable for them.