How do I send an invoice in a different language than my account?

Every invoice and estimate has its own language setting, independent of your account language. Open the document, click the document details area (where the issue date and number live), and pick the language. The PDF labels, the public view, and any email template you use are then rendered in that language.

If you regularly invoice the same customer in another language, set it once on the customer's profile instead of per-document — see Setting defaults for individual customers. Every new invoice for that customer then picks up the right language automatically.