Looking at the en.json file, I think it would be necessary to flatten it. So instead of a nested structure like you have, the strings should all be at the same level.

Flattening isn't necessary, as long as the values are only objects or strings. We just need to reserve one character as a separator that does not appear in the keys otherwise for the automatic flattening.

Oh ok, that's good!

Change 491788 had a related patch set uploaded (by Nikerabbit; owner: Nikerabbit):
[translatewiki@master] Add Wikidocumentaries

https://gerrit.wikimedia.org/r/491788

Patch is ready, but before I merge it: there is Finnish in https://github.com/Wikidocumentaries/wikidocumentaries-ui/blob/master/src/store/translation/en.json#L38 and I'd like to confirm that you are going to add message documentation.

Should be ready now

Change 491788 merged by jenkins-bot:
[translatewiki@master] Add Wikidocumentaries

https://gerrit.wikimedia.org/r/491788

https://translatewiki.net/w/i.php?title=Special:Translate&group=wikidocumentaries
Please expand https://translatewiki.net/wiki/Translating:Wikidocumentaries

I also need a short blurb for https://translatewiki.net/wiki/Group_descriptions

And please fix: No source text for ui-topic_page.TopicTimeline.centuryText (wikidocumentaries) https://github.com/Wikidocumentaries/wikidocumentaries-ui/blob/master/src/store/translation/en.json#L79

Should be done now. Are the existing translations preserved? They are in the same GitHub folder.

Yes they have been preserved as you can see from https://translatewiki.net/wiki/Translating:Wikidocumentaries translation statistics.