The program’s interface with the user should be designed to ease the user’s task. One way to ease the user’s task is to use messages in whatever language the user prefers.
Printing messages in different languages can be implemented in different ways. One could add all the different languages in the source code and choose among the variants every time a message has to be printed. This is certainly not a good solution since extending the set of languages is cumbersome (the code must be changed) and the code itself can become really big with dozens of message sets.
A better solution is to keep the message sets for each language in separate files which are loaded at runtime depending on the language selection of the user.
The GNU C Library provides two different sets of functions to support
message translation. The problem is that neither of the interfaces is
officially defined by the POSIX standard. The catgets family of
functions is defined in the X/Open standard but this is derived from
industry decisions and therefore not necessarily based on reasonable
decisions.
As mentioned above, the message catalog handling provides easy extendability by using external data files which contain the message translations. I.e., these files contain for each of the messages used in the program a translation for the appropriate language. So the tasks of the message handling functions are
The two approaches mainly differ in the implementation of this last step. Decisions made in the last step influence the rest of the design.