Using "ca" is great - that's the ISO 639-1 2-char language code.

Do I need to make "the whole translation in a shot"? I mean: I'd like to progressively add rows (maybe 5 per day), can't do it in a row. Does it behave well if I set "only a few keys"?
I'd be expenting that a key found in ca.json is used but if not found, it takes the default, so I can translate progressively.

That would be nice! But at the moment, no - that's not hooked up. To be honest, the translations are pretty basic right now - there were other priorities for the script, so getting things translated is a pretty manual process.

What I'd suggest doing it making a copy of the client/src/i18n/en.json file and call it to ca.json, then you can translate the contents at your leisure.

Similarly, all the various plugins will need a separate file too...

If you like, I could add a grunt command to at least do this for you? i.e. create copies of the English file with the new filename in all folders. Otherwise it'll be kind of a pain. Just let me know.

Oh - and lastly, you'll need to add the new ca value to GD_LOCALES in .env.default.

GD_LOCALES=ar,de,en,es,fr,hi,ja,nl,pt,ta,zh

That'll then be picked up by the build and be recognized in the interface.