How can I generate an Entity-Relationship Diagram and SQL DDL statements from natural language text?
There are many ways to do this, but the most important thing to know is that there is a name for what you want to do: Object Role Modeling. It's a process for creating conceptual models using natural-language semantics.

This is a pretty good (if old) overview of the process.

The Wikipedia page for ORM has a pretty complete list of the available tools, many of which are free.

The most up-to-date solution is NORMA, since it is actively maintained by the ORM Foundation, but that's a plug-in that requires Visual Studio.

If you don't have Visual Studio, Infagon looks like it is still pretty actively developed and may be your best bet.

CaseTalk is also worth a look. Free to students and educators, but commercial otherwise.