The translatable tag was an attempt to make certain things that weren't translatable translatable by providing a schema and query flag/tag that would all contrib to alter things queries and e.g. join a table that contains the translation.
I think I've written the only implementation of that tag and while it kinda works, it is too limited to actually work. There is no way to control when the original should be displayed and when it should be translated into which language.
I think the two main approaches for translating things are now content/entity translation and config translation.
