Add custom block option to follow the translation system easily

Follow-up to #2956 (Translate custom blocks): Custom blocks should get an option to just follow the translation system without having to set up complicated visibility conditions. Here are two examples why the option would be helpful:

(1) To display a custom block on a particular page, I use to add a visibility condition like Current path is one of "node/3". With a translated block I have to mention the paths of the translated nodes as well, e.g.: Current path is one of "node/3, node/4 , node/5". In case of many translations, an option to only mention the source node (which then had to follow the translation system) would be helpful.

(2) When I place a custom block on a layout where I normally don't have to add any visibility condition, e.g. on the Home page layout, I have to do so anyway for a translated / translatable block, e.g. Site language is one of "German, French, English". One has to consider here which languages to enable (if all, or only the ones already holding content for this block). That might be quite fine, but one has to reconsider these questions when new languages are added to the site or when translations are added to this block. All in all, it would be more easy to just tick an option in the block interface like "Follow the translation system". As an alternative, the additional option Current user's language in the Language visibility condition, comparable to the Views filter option, would also be helpful, but that wouldn't solve case (1) where the source node is to be identified as well.