The Title objects returned by EntityTitleLookup for foreign EntityIds should produce a title that refers to the respective target wiki. In order to achieve this, two things must be known:

1. the interwiki prefix for the target repo. This should, by convention, be the same as the repo name. But we'll need a mechanism to enforce this, or to provide a mapping in case this assumption doesn't hold.
2. the page title on the target wiki. This means we need to either know the namespace name for each entity type on the target wiki, or we need a way to link without knowing the namespace name. This could be achieved by each repo exposing Special:EntityPage, which will redirect to the correct local page given an entity ID.