feat(i18n): add course language config with 11 languages and UI linkage

- Add lib/i18n/course-languages.ts with curated language list
  (zh-CN, zh-TW, en-US, ja, ko, fr, de, es, pt, ru, ar) including
  native labels and English prompt names
- Course language defaults to UI locale on first visit; once user
  explicitly picks a language, that choice persists across sessions
- Replace toggle button with dropdown selector showing native labels
- Widen language types from 'zh-CN'|'en-US' to string throughout
- Fix hardcoded language ternaries in LLM prompt injection:
  - prompt-builder.ts: use getCourseLanguagePromptName()
  - classroom-generation.ts: remove normalizeLanguage() that forced
    all non-English to zh-CN
  - PBL system prompt, agent templates, generate-pbl: append language
    instruction for non-zh/en languages
  - quiz-grade API: add language suffix for grading feedback

Closes #327

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/api/quiz-grade/route.ts

@@ -10,6 +10,7 @@ import { callLLM } from '@/lib/ai/llm';
 import { createLogger } from '@/lib/logger';
 import { apiError, apiSuccess } from '@/lib/server/api-response';
 import { resolveModelFromHeaders } from '@/lib/server/resolve-model';
+import { getCourseLanguagePromptName } from '@/lib/i18n/course-languages';
 const log = createLogger('Quiz Grade');
 
 interface GradeRequest {
@@ -38,6 +39,9 @@ export async function POST(req: NextRequest) {
     const { model: languageModel } = resolveModelFromHeaders(req);
 
     const isZh = language === 'zh-CN';
+    const langName = getCourseLanguagePromptName(language || 'en-US');
+    const langSuffix =
+      !isZh && language !== 'en-US' ? `\nIMPORTANT: Write your comment in ${langName}.` : '';
 
     const systemPrompt = isZh
       ? `你是一位专业的教育评估专家。请根据题目和学生答案进行评分并给出简短评语。
@@ -45,7 +49,7 @@ export async function POST(req: NextRequest) {
 {"score": <0到${points}的整数>, "comment": "<一两句评语>"}`
       : `You are a professional educational assessor. Grade the student's answer and provide brief feedback.
 You must reply in the following JSON format only (no other content):
-{"score": <integer from 0 to ${points}>, "comment": "<one or two sentences of feedback>"}`;
+{"score": <integer from 0 to ${points}>, "comment": "<one or two sentences of feedback>"}${langSuffix}`;
 
     const userPrompt = isZh
       ? `题目：${question}

app/page.tsx

@@ -46,6 +46,7 @@ import { toast } from 'sonner';
 import { Tooltip, TooltipContent, TooltipTrigger } from '@/components/ui/tooltip';
 import { useDraftCache } from '@/lib/hooks/use-draft-cache';
 import { SpeechButton } from '@/components/audio/speech-button';
+import { findClosestCourseLanguage } from '@/lib/i18n/course-languages';
 
 const log = createLogger('Home');
 
@@ -56,7 +57,7 @@ const RECENT_OPEN_STORAGE_KEY = 'recentClassroomsOpen';
 interface FormState {
   pdfFile: File | null;
   requirement: string;
-  language: 'zh-CN' | 'en-US';
+  language: string;
   webSearch: boolean;
 }
 
@@ -99,11 +100,12 @@ function HomePage() {
       const savedLanguage = localStorage.getItem(LANGUAGE_STORAGE_KEY);
       const updates: Partial<FormState> = {};
       if (savedWebSearch === 'true') updates.webSearch = true;
-      if (savedLanguage === 'zh-CN' || savedLanguage === 'en-US') {
+      if (savedLanguage) {
+        // User previously made an explicit choice — respect it
         updates.language = savedLanguage;
       } else {
-        const detected = navigator.language?.startsWith('zh') ? 'zh-CN' : 'en-US';
-        updates.language = detected;
+        // First visit: derive course language from UI locale
+        updates.language = findClosestCourseLanguage(locale);
       }
       if (Object.keys(updates).length > 0) {
         setForm((prev) => ({ ...prev, ...updates }));

components/generation/generation-toolbar.tsx

@@ -21,6 +21,7 @@ import type { WebSearchProviderId } from '@/lib/web-search/types';
 import type { ProviderId } from '@/lib/ai/providers';
 import type { SettingsSection } from '@/lib/types/settings';
 import { MediaPopover } from '@/components/generation/media-popover';
+import { courseLanguages, getCourseLanguageLabel } from '@/lib/i18n/course-languages';
 
 // ─── Constants ───────────────────────────────────────────────
 const MAX_PDF_SIZE_MB = 50;
@@ -28,8 +29,8 @@ const MAX_PDF_SIZE_BYTES = MAX_PDF_SIZE_MB * 1024 * 1024;
 
 // ─── Types ───────────────────────────────────────────────────
 export interface GenerationToolbarProps {
-  language: 'zh-CN' | 'en-US';
-  onLanguageChange: (lang: 'zh-CN' | 'en-US') => void;
+  language: string;
+  onLanguageChange: (lang: string) => void;
   webSearch: boolean;
   onWebSearchChange: (v: boolean) => void;
   onSettingsOpen: (section?: SettingsSection) => void;
@@ -357,19 +358,36 @@ export function GenerationToolbar({
         </Tooltip>
       )}
 
-      {/* ── Language pill ── */}
-      <Tooltip>
-        <TooltipTrigger asChild>
-          <button
-            onClick={() => onLanguageChange(language === 'zh-CN' ? 'en-US' : 'zh-CN')}
-            className={pillMuted}
-          >
-            <Globe className="size-3.5" />
-            <span>{language === 'zh-CN' ? '中文' : 'EN'}</span>
-          </button>
-        </TooltipTrigger>
-        <TooltipContent>{t('toolbar.languageHint')}</TooltipContent>
-      </Tooltip>
+      {/* ── Language selector ── */}
+      <Popover>
+        <Tooltip>
+          <TooltipTrigger asChild>
+            <PopoverTrigger asChild>
+              <button className={pillMuted}>
+                <Globe className="size-3.5" />
+                <span>{getCourseLanguageLabel(language)}</span>
+              </button>
+            </PopoverTrigger>
+          </TooltipTrigger>
+          <TooltipContent>{t('toolbar.languageHint')}</TooltipContent>
+        </Tooltip>
+        <PopoverContent className="w-40 p-1" align="start">
+          {courseLanguages.map((lang) => (
+            <button
+              key={lang.code}
+              onClick={() => onLanguageChange(lang.code)}
+              className={cn(
+                'flex w-full items-center justify-between rounded-sm px-2 py-1.5 text-sm transition-colors hover:bg-muted/60',
+                language === lang.code && 'font-medium text-foreground',
+                language !== lang.code && 'text-muted-foreground',
+              )}
+            >
+              <span>{lang.label}</span>
+              {language === lang.code && <Check className="size-3.5" />}
+            </button>
+          ))}
+        </PopoverContent>
+      </Popover>
 
       {/* ── Separator ── */}
       <div className="w-px h-4 bg-border/60 mx-1" />

lib/generation/prompts/templates/requirements-to-outlines/user.md

@@ -14,7 +14,7 @@ Please generate scene outlines based on the following course requirements.
 
 **Required language**: {{language}}
 
-(If language is zh-CN, all content must be in Chinese; if en-US, all content must be in English)
+(All content must be generated in the specified language)
 
 ---
 

lib/generation/scene-generator.ts

@@ -735,7 +735,7 @@ function normalizeQuizAnswer(question: Record<string, unknown>): string[] | unde
 async function generateInteractiveContent(
   outline: SceneOutline,
   aiCall: AICallFn,
-  language: 'zh-CN' | 'en-US' = 'zh-CN',
+  language: string = 'zh-CN',
 ): Promise<GeneratedInteractiveContent | null> {
   const config = outline.interactiveConfig!;
 

lib/i18n/course-languages.ts

@@ -0,0 +1,59 @@
+/**
+ * Curated list of languages available for course generation.
+ *
+ * To add a new language, append an entry here — no other file changes needed.
+ * - code:       BCP-47 tag stored in settings / passed to APIs
+ * - label:      Native name shown in the UI (autoglossonym, no i18n needed)
+ * - promptName: English name injected into LLM prompts
+ */
+export const courseLanguages = [
+  { code: 'zh-CN', label: '简体中文', promptName: 'Chinese (Simplified)' },
+  { code: 'zh-TW', label: '繁體中文', promptName: 'Chinese (Traditional)' },
+  { code: 'en-US', label: 'English', promptName: 'English' },
+  { code: 'ja', label: '日本語', promptName: 'Japanese' },
+  { code: 'ko', label: '한국어', promptName: 'Korean' },
+  { code: 'fr', label: 'Français', promptName: 'French' },
+  { code: 'de', label: 'Deutsch', promptName: 'German' },
+  { code: 'es', label: 'Español', promptName: 'Spanish' },
+  { code: 'pt', label: 'Português', promptName: 'Portuguese' },
+  { code: 'ru', label: 'Русский', promptName: 'Russian' },
+  { code: 'ar', label: 'العربية', promptName: 'Arabic' },
+] as const;
+
+export type CourseLanguageCode = (typeof courseLanguages)[number]['code'];
+
+const defaultCode: CourseLanguageCode = 'zh-CN';
+
+/** Get the native display label for a course language code. */
+export function getCourseLanguageLabel(code: string): string {
+  return courseLanguages.find((l) => l.code === code)?.label ?? code;
+}
+
+/** Get the English prompt name for a course language code. */
+export function getCourseLanguagePromptName(code: string): string {
+  return courseLanguages.find((l) => l.code === code)?.promptName ?? code;
+}
+
+/**
+ * Find the closest supported course language for a given locale string.
+ * Matching order: exact code → base language prefix → default (zh-CN).
+ *
+ * Examples:
+ *   'en-US' → 'en-US'   (exact)
+ *   'en-GB' → 'en-US'   (prefix 'en' matches 'en-US')
+ *   'zh-TW' → 'zh-TW'   (exact)
+ *   'zh'    → 'zh-CN'   (prefix 'zh' matches 'zh-CN', first in list)
+ *   'sv'    → 'zh-CN'   (no match, fallback)
+ */
+export function findClosestCourseLanguage(locale: string): CourseLanguageCode {
+  // Exact match
+  const exact = courseLanguages.find((l) => l.code === locale);
+  if (exact) return exact.code;
+
+  // Base language prefix match (e.g. 'en' from 'en-GB')
+  const base = locale.split('-')[0];
+  const prefixMatch = courseLanguages.find((l) => l.code.split('-')[0] === base);
+  if (prefixMatch) return prefixMatch.code;
+
+  return defaultCode;
+}

lib/orchestration/prompt-builder.ts

@@ -8,6 +8,7 @@ import type { StatelessChatRequest } from '@/lib/types/chat';
 import type { AgentConfig } from '@/lib/orchestration/registry/types';
 import type { WhiteboardActionRecord, AgentTurnSummary } from './director-prompt';
 import { getActionDescriptions, getEffectiveActions } from './tool-schemas';
+import { getCourseLanguagePromptName } from '@/lib/i18n/course-languages';
 
 // ==================== Role Guidelines ====================
 
@@ -166,7 +167,7 @@ Personalize your teaching based on their background when relevant. Address them
   // Build language constraint from stage language
   const courseLanguage = storeState.stage?.language;
   const languageConstraint = courseLanguage
-    ? `\n# Language (CRITICAL)\nYou MUST speak in ${courseLanguage === 'zh-CN' ? 'Chinese (Simplified)' : courseLanguage === 'en-US' ? 'English' : courseLanguage}. ALL text content in your response MUST be in this language.\n`
+    ? `\n# Language (CRITICAL)\nYou MUST speak in ${getCourseLanguagePromptName(courseLanguage)}. ALL text content in your response MUST be in this language.\n`
     : '';
 
   return `# Role

lib/pbl/generate-pbl.ts

@@ -18,6 +18,7 @@ import { ProjectMCP } from './mcp/project-mcp';
 import { AgentMCP } from './mcp/agent-mcp';
 import { IssueboardMCP } from './mcp/issueboard-mcp';
 import { buildPBLSystemPrompt } from './pbl-system-prompt';
+import { getCourseLanguagePromptName } from '@/lib/i18n/course-languages';
 import type { PBLMode } from './types';
 
 export interface GeneratePBLConfig {
@@ -290,7 +291,7 @@ export async function generatePBLContent(
       prompt:
         language === 'zh-CN'
           ? `请设计一个PBL项目。现在从 project_info 模式开始，先设置项目标题和描述。`
-          : `Design a PBL project. Start in project_info mode by setting the project title and description.`,
+          : `Design a PBL project. Start in project_info mode by setting the project title and description.${language !== 'en-US' ? ` Respond in ${getCourseLanguagePromptName(language)}.` : ''}`,
       tools: pblTools,
       stopWhen: stepCountIs(30),
       onStepFinish: ({ toolCalls, text }) => {
@@ -395,7 +396,7 @@ Based on the issue information above, generate 1-3 specific, actionable question
 - Help break down the problem
 - Encourage critical thinking
 
-Format your response as a numbered list.`;
+Format your response as a numbered list.${language !== 'en-US' ? `\n\nIMPORTANT: Generate all questions in ${getCourseLanguagePromptName(language)}.` : ''}`;
 
     const questionResult = await callLLM(
       {

lib/pbl/mcp/agent-templates.ts

@@ -4,18 +4,26 @@
  * Migrated from PBL-Nano with multi-language support.
  */
 
+import { getCourseLanguagePromptName } from '@/lib/i18n/course-languages';
+
+function appendLanguageInstruction(prompt: string, language: string): string {
+  if (language === 'zh-CN' || language === 'en-US') return prompt;
+  const langName = getCourseLanguagePromptName(language);
+  return `${prompt}\n\n## Language (CRITICAL)\nYou MUST respond in ${langName}.`;
+}
+
 export function getQuestionAgentPrompt(language: string = 'en-US'): string {
   if (language === 'zh-CN') {
     return QUESTION_AGENT_TEMPLATE_PROMPT_ZH;
   }
-  return QUESTION_AGENT_TEMPLATE_PROMPT;
+  return appendLanguageInstruction(QUESTION_AGENT_TEMPLATE_PROMPT, language);
 }
 
 export function getJudgeAgentPrompt(language: string = 'en-US'): string {
   if (language === 'zh-CN') {
     return JUDGE_AGENT_TEMPLATE_PROMPT_ZH;
   }
-  return JUDGE_AGENT_TEMPLATE_PROMPT;
+  return appendLanguageInstruction(JUDGE_AGENT_TEMPLATE_PROMPT, language);
 }
 
 export const QUESTION_AGENT_TEMPLATE_PROMPT = `You are a Question Agent in a Project-Based Learning platform. Your role is to help students understand and complete their assigned issue.

lib/pbl/pbl-system-prompt.ts

@@ -5,6 +5,8 @@
  * Enhanced with multi-language support and configurable parameters.
  */
 
+import { getCourseLanguagePromptName } from '@/lib/i18n/course-languages';
+
 export interface PBLSystemPromptConfig {
   projectTopic: string;
   projectDescription: string;
@@ -20,6 +22,12 @@ export function buildPBLSystemPrompt(config: PBLSystemPromptConfig): string {
     return buildPBLSystemPromptZH(config);
   }
 
+  const langName = getCourseLanguagePromptName(language);
+  const langInstruction =
+    language !== 'en-US'
+      ? `\n\n## Language (CRITICAL)\nYou MUST produce ALL output in ${langName}. Every title, description, role name, issue text, and any other generated content MUST be in ${langName}.\n`
+      : '';
+
   return `You are a Teaching Assistant (TA) on a Project-Based Learning platform. You are fully responsible for designing group projects for students based on the course information provided by the teacher.
 
 ## Your Responsibility
@@ -81,7 +89,7 @@ When you create issues:
 
 **IMPORTANT**: Once you have configured the project info, defined all necessary agents (roles), and created the issueboard with tasks, you MUST set your mode to **idle** to indicate completion.
 
-Your initial mode is **project_info**.`;
+Your initial mode is **project_info**.${langInstruction}`;
 }
 
 function buildPBLSystemPromptZH(config: PBLSystemPromptConfig): string {

lib/server/classroom-generation.ts

@@ -96,8 +96,8 @@ function createInMemoryStore(stage: Stage): StageStore {
   };
 }
 
-function normalizeLanguage(language?: string): 'zh-CN' | 'en-US' {
-  return language === 'en-US' ? 'en-US' : 'zh-CN';
+function normalizeLanguage(language?: string): string {
+  return language || 'zh-CN';
 }
 
 function stripCodeFences(text: string): string {

lib/types/generation.ts

@@ -64,7 +64,7 @@ export interface UploadedDocument {
  */
 export interface UserRequirements {
   requirement: string; // Single free-form text for all user input
-  language: 'zh-CN' | 'en-US'; // Course language - critical for generation
+  language: string; // Course language - critical for generation
   userNickname?: string; // Student nickname for personalization
   userBio?: string; // Student background for personalization
   webSearch?: boolean; // Enable web search for richer context
@@ -100,7 +100,7 @@ export interface SceneOutline {
   teachingObjective?: string;
   estimatedDuration?: number; // seconds
   order: number;
-  language?: 'zh-CN' | 'en-US'; // Generation language (inherited from requirements)
+  language?: string; // Generation language (inherited from requirements)
   // Suggested image IDs (from PDF-extracted images)
   suggestedImageIds?: string[]; // e.g., ["img_1", "img_3"]
   // AI-generated media requests (when PDF images are insufficient)
@@ -124,7 +124,7 @@ export interface SceneOutline {
     projectDescription: string;
     targetSkills: string[];
     issueCount?: number;
-    language: 'zh-CN' | 'en-US';
+    language: string;
   };
 }