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The Eiffel mechanism for interfacing with other languages, as defined by the Eiffel standard (8.31) uses a syntax for external features, that is, features that are in languages or libraries external to Eiffel.
Additionally, there is a notion of such external languages being either registered or unregistered. A language is registered if every Eiffel implementation must support the external interface to this language as defined by the standard. So the names of all registered languages appear in the Eiffel standard. And the details of the specific external mechanisms for those languages also appear in the standard. Registered languages included in the June 2006 edition of the standard are "C", "C++", and "dll".
The case of unregistered languages is simply that there is no guarantee that such a language is supported by any particular Eiffel compiler. So, a the name of an unregistered language might appear as "Cobol" or "Ada". These names may be meaningful to particular Eiffel compilers, but from the standpoint of the standard they are arbitrary manifest strings.