Message catalogs a la X/Open (The GNU C Library)

### 8.1 X/Open Message Catalog Handling ¶

The `catgets` functions are based on the simple scheme:

> Associate every message to translate in the source code with a unique identifier. To retrieve a message from a catalog file solely the identifier is used.

This means for the author of the program that s/he will have to make sure the meaning of the identifier in the program code and in the message catalogs is always the same.

Before a message can be translated the catalog file must be located. The user of the program must be able to guide the responsible function to find whatever catalog the user wants. This is separated from what the programmer had in mind.

All the types, constants and functions for the `catgets` functions are defined/declared in the `nl_types.h` header file.