After you have created a message catalog, you can use these functions.
The C function CATOPEN opens the message catalog. If no slash (/) characters are found in the name, the NLSPATH environment variable and the LC_MESSAGES category are used to find the specified message catalog. If the name contains one or more slash (/) characters, the name is interpreted as a path name of the catalog to open.
A default path is used if there is no NLSPATH environment variable or a message catalog cannot be found in the NLSPATH path specified. If the value of oflag is NO_CAT_LOCALE the environment variable setting of LC_MESSAGES may affect the default path. If the value of oflag is zero the LANG environment variable may affect it also.
The WebSphere® Development Studio: ILE C/C++ Language Reference PDF has more information about C functions and message catalogs..