Can I have different invoice templates with different languages?
Example: Can I have a French template and an English template and choose which one to use for each client?
No. 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.
One workaround here would be to enter both languages into the fields of your 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.