In a follow up to – WPML: Translating blocks that aren’t translatable
The ALT text in Cover block images was not translatable for me. Here’s the XML to add to the WMP->Settings->Custom XML Configuration panel to enable it. This makes both the ALT in the image tag and in the HTML comment translatable.
<wpml-config>
<gutenberg-blocks>
<gutenberg-block type="core/cover" translate="1">
<key name="alt" />
<xpath>//img/@alt</xpath>
</gutenberg-block>
</gutenberg-blocks>
</wpml-config>