When you create an invoice, you should have an option on the right hand side:

Simply click ‘English’ (or whatever yours says), and you should be presented with a drop down of options:

When you view the invoice, you will notice the language would have changed. For example, in Welsh, my invoice looks like this:
Edit
Just a quick note, if the language you want isn’t supported yet, you can always customise the the labels yourself:
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