https://github.com/hexchat/hexchat/blob/master/src/common/cfgfiles.c#L707 defaults hex_gui_lang to 15 (en) instead of honoring system locale. This means fresh installs (no existing config) are always set to English.
Looks like the only fix is to mirror the gigantic switch case in hexchat.c to set the pref according to system locale.
Now the default configuration tries to set the language from the system
locale, and defaults to english if no language match.
This closes #473.
Still affects Windows.