po file are handled by gettext/dgettext/dcgettext.

1. The encoding of po file is specified in po file through 'charset' keyword, gnome messages files are utf8.
2. The output encoding of gettext is dependent to LC_CTYPE at default
3. You can use bind_textdomain_codeset() to specify what encoding of the output of gettext is.

char * gettext(const char *msgid);

char * dgettext(const char *domainname, const char * msgid);

char * dcgettext(const char *domainname, const char *msgid, int category);

1. What is message domain?
   1. A set of translatabel msgid messages.
   2. Use domain name to determine where the translation is looked up.
2. The message domain of gettext is specified by a preceding textdomain call.
3. Translation Operation
   1. gettext, dgettext use LC_MESSAGES to lookup. It is determined by a preceding call to the setlocale function. setlocale(LC_ALL, "") initializes the LC_MESSAGE locale based on the first nonempty value of the three enviroment variabled LC_ALL, LC_MESSAGES, LANG.
   2. Use LC_CTYPE to convert the translated message from the translator's codeset(specified in po file, mostly is UTF-8) to the current locale's codeset. You can use bint_textdomain_codeset to specify the output messages encoding.