Add a translation guide for contributors

[flpm]

This PR adds a translation guide to the Package Guide to help contributors get started contributing to the translation process.

The nox sessions described in the guide in this PR are defined in PR #298 (Add support for internationalization to the guide)

What was done

I wrote a first draft of a Translation Guide (TRANSLATING.md) similar to the CONTRIBUTING.md guide to help new contributors understand the Translation workflow and get started with the process.

It assumes the reader is on the beginner end of the technical spectrum (translation is a good opportunity for people new to open source) and does not necessarily know internationalization and localization, so it is a little on the verbose side.

This guide will also help the pyOpenSci community involved in the guide understand the changes proposed in PR #298, which is where the workflow described here is implemented.

In this initial draft I suggest some ideas that the pyOpenSci community should discuss and decide, they come from similar translation processes in other projects.

For example:

• What will be the review process for a translation PR? Will there be one or more coordinators/editors per language in charge of the review?
• When will a translation be released to the site? As soon as it is started? After a certain amount is translated? Once it is finished? Untranslated strings will appear in English.
• Should PRs for translations be tagged (e.g., lang-es to help the coordinators/editors identify them? This could be automated in Github Actions.

[lwasser]

@flpm i'm scrambling to get ready for scipy but my intent is to look at your PR's today / monday and get some things merged. THANK YOU for all of this work! i'm just a bit behind with all of the travel! I really suspect we can get some folks involved at SciPy with looking at these translations which will be amazing. it's the absolute perfect beginner-friendly sprint task for someone to work on that would have huge impact .

[lwasser]

ok this is all fantastic! I am going to merge it Filipe but also i want to open a new issue around our contributor guide. We might be able to pull a bit of the language from your guide here into our main contributor guide and cross link.