i18n: built-in zh fallback + fix setting lang URL + localize bare help

- pmm_tr now falls back to a built-in zh-CN string table when no .pjson is
  installed, so output is readable instead of printing raw keys (msg.xxx).
- pmm_lang_load_active() loads the built-in zh if the on-disk pack is absent.
- main(): initialise i18n BEFORE the bare-arg print_help, so 'pmm' help is
  localized too (previously it ran before the language pack was loaded).
- pmm setting lang <loc>: the download URL now points to {host}/mirror/lang/
  (from the registry base, swapping '/packages' for '/lang'), fixing the
  'download-failed' for language packs.

src/i18n.c

@@ -43,13 +43,39 @@ int pmm_lang_load(const char *path) {
     return 0;
 }
 
+/* Built-in fallback so PMM is usable even before any .pjson is installed. */
+static const char *kBuiltinZh =
+"{\"help.usage\":\"用法:\",\"help.commands\":\"命令:\",\"help.options\":\"选项:\","
+"\"cmd.install\":\"install\",\"cmd.remove\":\"remove\",\"cmd.update\":\"update\","
+"\"cmd.upgrade\":\"upgrade\",\"cmd.mirror\":\"mirror\",\"cmd.search\":\"search\","
+"\"msg.looking-up\":\"在镜像 %s 中查找 %s\",\"msg.selected\":\"已选择 %s@%s (os=%s arch=%s)\","
+"\"msg.downloading\":\"下载 %s\",\"msg.downloaded\":\"已下载 %s\","
+"\"msg.hash-sha256\":\"sha256: %s\",\"msg.hash-sha1\":\"sha1: %s\","
+"\"msg.installing\":\"正在安装 %s ...\",\"msg.installed\":\"已安装 %s\","
+"\"msg.checksum-ok\":\"%s 校验 OK: %s\",\"msg.checksum-mismatch\":\"校验不匹配 (%s)!\","
+"\"msg.resolving-dep\":\"正在解析依赖 %s\",\"msg.registry-ok\":\"注册表 sha256 OK: %s\","
+"\"msg.path-exists\":\"PATH 已包含 PMM 目录\",\"msg.err.registry-not-found\":\"找不到软件包 '%s'\","
+"\"msg.err.usage\":\"用法\",\"msg.err.unknown-cmd\":\"未知命令 '%s' (试试 'pmm help')\","
+"\"msg.err.download-failed\":\"下载语言包 %s 失败\",\"msg.err.invalid-locale\":\"无效语言 %s\","
+"\"msg.err.not-found\":\"找不到软件包 '%s'\",\"msg.err.failed-install\":\"安装 %s 失败\","
+"\"msg.err.no-registry-mirror\":\"没有配置注册表镜像\",\"msg.err.cannot-write\":\"无法写入 %s\","
+"\"desc.install\":\"安装\",\"desc.remove\":\"卸载\",\"desc.update\":\"刷新索引\","
+"\"desc.upgrade\":\"升级\",\"desc.mirror\":\"镜像源\",\"desc.search\":\"搜索\","
+"\"desc.info\":\"详情\",\"desc.self-update\":\"更新自身\",\"desc.help\":\"帮助\","
+"\"opt.p-drive\":\"安装盘符\",\"opt.no-color\":\"禁用颜色\",\"opt.quiet\":\"静默\","
+"\"opt.verbose\":\"详情\"}";
+
 int pmm_lang_load_active(void) {
     char dir[1024];
     pmm_config_dir(dir, sizeof(dir));
     char path[1400];
     snprintf(path, sizeof(path), "%s/lang/%s.pjson", dir, pmm_lang_locale());
     if (pmm_lang_load(path) == 0) return 0;
-    return 1;   /* no pack on disk */
+    /* No pack on disk: fall back to the built-in zh-CN so output is readable
+     * instead of printing raw keys (msg.xxx). */
+    JsonValue *root = json_parse(kBuiltinZh);
+    if (root) { if (g_lang) json_free(g_lang); g_lang = root; return 0; }
+    return 1;
 }
 
 const char *pmm_tr(const char *key) {

src/main.c

@@ -934,8 +934,23 @@ static int cmd_setting(int argc, char **argv) {
         if (ml->items[i].registry && *ml->items[i].registry) { base = ml->items[i].registry; break; }
     if (!base) { pmm_error("%s", pmm_tr("msg.err.no-registry-mirror")); mirrors_free(ml); return 1; }
 
+    /* Language packs live under {host}/mirror/lang/, NOT under the registry
+     * base {host}/mirror/packages. Derive it by swapping the trailing
+     * "/packages" (if present) for "/lang". */
+    char base2[2048];
+    snprintf(base2, sizeof(base2), "%s", base);
+    {
+        size_t bl = strlen(base2);
+        static const char *pkgsuf = "/packages";
+        size_t pl = strlen(pkgsuf);
+        if (bl >= pl && strcmp(base2 + bl - pl, pkgsuf) == 0) {
+            strcpy(base2 + bl - pl, "/lang");   /* .../mirror/packages -> .../mirror/lang */
+        } else {
+            snprintf(base2 + bl, sizeof(base2) - bl, "/lang");
+        }
+    }
     char url[2048];
-    snprintf(url, sizeof(url), "%s/lang/%s.pjson", base, loc);
+    snprintf(url, sizeof(url), "%s/%s.pjson", base2, loc);
     mkdir_p_local(langdir);
     char out[1400];
     snprintf(out, sizeof(out), "%s/%s.pjson", langdir, loc);
@@ -1049,19 +1064,20 @@ int main(int argc, char **argv) {
         }
     }
 #endif
-    if (argc < 2) { print_help(); return 0; }
-    pmm_set_self_path(argv[0]);
-    argc = consume_drive_flag(argc, argv);
-    argc = consume_global_flags(argc, argv);
-    if (argc < 2) { print_help(); return 0; }
 
-    /* initialise i18n: use configured language (if any), then load its pack */
+    /* initialise i18n BEFORE any output (so `pmm` bare help is localized too).
+     * Use the configured language (if any), then load its pack (or built-in). */
     {
         PmmConfig cfg; load_config(&cfg);
         if (cfg.language && *cfg.language) pmm_lang_set_locale(cfg.language);
         free(cfg.registry_url); free(cfg.mirror_name); free(cfg.language);
         pmm_lang_load_active();
     }
+    if (argc < 2) { print_help(); return 0; }
+    pmm_set_self_path(argv[0]);
+    argc = consume_drive_flag(argc, argv);
+    argc = consume_global_flags(argc, argv);
+    if (argc < 2) { print_help(); return 0; }
 
     if (strcmp(argv[1], "help") == 0 || strcmp(argv[1], "--help") == 0 || strcmp(argv[1], "-h") == 0) {
         print_help(); return 0;