How can I generate an Entity-Relationship Diagram and SQL DDL statements from natural language text? I am using LaTeX or another text-macro tool/library. How can I (1) use natural language text to describe relationships, then (2) generate an Entity-Relationship Diagram and (3) generate SQL DDL to create the database schema from it?
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.