I've managed to figure out that reusable custom blocks can be translated, if you create or edit them from Structure > Custom Blocks. However, when I add one of these translated blocks to the layout, it doesn't seem to switch automatically when I switch languages using the Language Switcher. Why is that?
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I think the confusion came in because I first created a custom block directly on the Layout page. Then when I couldn't find the translate options, I saved it and went to the Structure > Custom Blocks page. I tried to edit the custom block I had just created and already placed on the Layout, and it seems that when I tried to translate it, it created a new copy of that block that was multilingual and allowed me to translate it.
The original (non-multilingual) block was still placed on the Layout so I had to remove that one and place the new multilingual version.
This UI can definitely be improved. I'll add my experience to this open issue in the Github queue.
I think the confusion came in because I first created a custom block directly on the Layout page. Then when I couldn't find the translate options, I saved it and went to the Structure > Custom Blocks page. I tried to edit the custom block I had just created and already placed on the Layout, and it seems that when I tried to translate it, it created a new copy of that block that was multilingual and allowed me to translate it.
The original (non-multilingual) block was still placed on the Layout so I had to remove that one and place the new multilingual version.
This UI can definitely be improved. I'll add my experience to this open issue in the Github queue.