Great that you want to contribute!
At the bottom of section 8 of the
invoice manual, you'll find
Whenever you create your own foreign language extension please kindly consider sending your modified
invoice.def file to the author (at
[email protected] or
[email protected]) so that it can be included in the next update. Future users can then share your work.
So, in this case (actually, in most cases), the documentation contains instructions about what you should do if you want to contribute to the package.
So to make it more general -- whenever we want to contribute to a translation we first read the package documentation. But if the info is hard to find? Do we contact the author then?
(quoted from your comment)
If the author/maintainer didn't leave any instructions about contribution anywhere, you should ask her/him for guidance. Authors/maintainers usually leave at least a contact email address in the documentation.
Whatever you do, always comply with the license that covers the work in question. In particular, if the package is under the LaTeX Project Public License (LPPL), the most popular license for TeX packages, you should not modify and then distribute the "Derived Work" under the same name without the author/maintainer's agreement.
And if you haven't managed to get any reply from the author/maintainer after six months and suspect that s/he has ceased maintenance of the package, you can follow the procedure for taking over maintenance of the package, which is detailed in the section entitled Maintenance of The Work of the LPPL.