i18n's routines are located in the languages::c file.

To use i18n in the server code, instead of putting strings, use

i18n(who, "This is a translatable message.");

with who the player's object you wish to translate for.

The string can be anything you want, and will get replaced by the localised version. They must not contain a | character. If the string doesn't exist for the language, the code will be returned.

The recommanded strategy for translation is:

use English sentences anywhere they are required, as usual but with the i18n call
translate them using the language files, but don't write the English (messages.en file) version
if the sentence contains a |, then use an arbitrary code, and do put it in the messages.en file

Language files are located in lib/i18n/messages.xx files, with xx the language's code, "en" or "fr" for instance.

They are installed as part of make install.

The format is straightforward:

any line starting with # is a comment
a line is in the format sentence code|localised version
if the line is simply sentence code, without any |, the localised version is the code
newlines can be written as \n
there must be a line with the code LN, which contains the native language's name
lines can contain printf format specifiers, it is your responsability to give the correct arguments

Warning: currently the order of arguments is fixed, as defined by the print call (English version if you followed this guide) in the code.