You can change the invoice and proposal fields to say whatever you'd like, in whatever language you'd like!
Just go to Settings > Label Translation and change the text for the fields you'd like to customize.
For example:
FROM | TO |
|
Unit Price | Hourly Rate | (if you charge based on an hourly rate) |
Quantity | Hours | (if you charge based on an hourly rate) |
Reference Number | Order Number |
|
Proposal | Purchase Order | more info here |