Clarify which Chinese translation we have

Chinese has several writing systems so we need to be clear which one
our translation covers. The one we currently have is for Traditional
Chinese so make sure it uses the matching language tag.

app/ui.js

@@ -1651,7 +1651,7 @@ var UI = {
 };
 
 // Set up translations
-var LINGUAS = ["de", "el", "es", "nl", "pl", "sv", "tr", "zh"];
+var LINGUAS = ["de", "el", "es", "nl", "pl", "sv", "tr", "zh_TW"];
 l10n.setup(LINGUAS);
 if (l10n.language !== "en" && l10n.dictionary === undefined) {
     WebUtil.fetchJSON('app/locale/' + l10n.language + '.json', function (translations) {

po/Makefile

@@ -1,7 +1,7 @@
 all:
 .PHONY: update-po update-js update-pot
 
-LINGUAS := de el es nl pl sv tr zh
+LINGUAS := de el es nl pl sv tr zh_TW
 
 VERSION := $(shell grep '"version"' ../package.json | cut -d '"' -f 4)