Hide language switcher block when there is only one language enabled

Before:

After:

I will also open an issue on core or search for an existing one. Because the list is empty ok, but the block is still present.

To avoid core patch.

I will try an approach based on hook_block_access, as this is where core does its logic to not display the block when there is only one language;

I have created a separated MR for the approach with hook_block_access.

I discovered that hook_block_access and hook_block_build_alter are not triggered when blocks are displayed in Layout Builder.

I think having a language switcher block rendered in Layout Builder is a very rare case (at least for now) but it may exist if for example you want to administer a header or footer with an entity like Config Page or maybe you use Page Manager.

Hence 2 MR so people can choose for now.

Uploading patches from MRs for easier Composer usage.

For Layout Builder block, an event subscriber listening to LayoutBuilderEvents::SECTION_COMPONENT_BUILD_RENDER_ARRAY should be implemented if needed.

Waiting for maintainers feedback.