Hello,
I'm coautor of Polish translation, I can only tell how it looks like from my perspective.
"So, after all, what is needed is a link to the deployed Go tour with a new translation, right?"
I would need to check that, since the other person take care of it.
"However, what is the right way to keep it up to date with the original repository?
What approach do most people use?"
Well, when I need current version of the repository, I just clone it using Git, change what is needed, fetch-merge-push to my fork of GitHub repository with polish translation and after that I make Pull Request for main repository of polish version and other person take care about that.
"Is there any described journey of adding a new translation?"
Can you explain what you mean by "journey"? It is not clear to me, what you want to say.
If something that I wrote is unclear, feel free to ask. It is not easy feat to find how much details you should put in the answer.
As a aside. From my perspective, the most consuming part was the translation of Go vocabulary into destination language. How you should translate things like "slice" or "channel"? Or even translation of "import path" can make a problem. Second thing was to smooth the translation. At first attempt it looks like someone took a dictionary and translate every word at once, which can result in very ackaword text. And since English text of Go tour is very compact, we try to make translation compat as well, which combined with demande that it will read naturaln, can be not easy.
Comparing to this, GitHub workflow was a piece of cake.
Best regards,
Kamil