I am a bit surprised that, with Odoo being a Belgian company, the software seems not to be compliant to the belgian legislation.
Odoo is based in the french speaking region of belgium, so invoices should be issued in the french language.
One is free to attach a translated invoice in a language the customer understands. (Such as english for example).
I have not found an easy way to do this without custom development. Or am I overlooking something ?