diff --git a/README.md b/README.md
index f6a5b48..1e8e5c6 100644
--- a/README.md
+++ b/README.md
@@ -10,7 +10,7 @@
 
 # 📄🔊 OpenReader
 
-OpenReader is an open source, self-host-friendly text-to-speech document reader built with Next.js for **EPUB, PDF, TXT, MD, and DOCX** with multilingual, synchronized read-along playback.
+OpenReader is an open-source, self-host-friendly text-to-speech document reader built with Next.js for **EPUB, PDF, TXT, MD, and DOCX** with multilingual, synchronized read-along playback.
 
 > Previously named **OpenReader-WebUI**.
 
diff --git a/docs-site/docs/introduction.md b/docs-site/docs/introduction.md
index a255ce3..8f60592 100644
--- a/docs-site/docs/introduction.md
+++ b/docs-site/docs/introduction.md
@@ -4,7 +4,7 @@ title: Introduction
 slug: /
 ---
 
-OpenReader is an open source text-to-speech document reader built with Next.js. It provides a multilingual read-along experience with narration for **EPUB, PDF, TXT, MD, and DOCX documents**.
+OpenReader is an open-source text-to-speech document reader built with Next.js. It provides a multilingual read-along experience with narration for **EPUB, PDF, TXT, MD, and DOCX documents**.
 
 > Previously named **OpenReader-WebUI**.
 
diff --git a/src/app/api/tts/segments/manifest/route.ts b/src/app/api/tts/segments/manifest/route.ts
index 686555c..f33155d 100644
--- a/src/app/api/tts/segments/manifest/route.ts
+++ b/src/app/api/tts/segments/manifest/route.ts
@@ -18,6 +18,7 @@ import type {
   TTSSegmentsManifestResponse,
 } from '@/types/client';
 import { isTtsProviderType } from '@/lib/shared/tts-provider-catalog';
+import { normalizeLanguageTag } from '@/lib/shared/language';
 import { resolveEffectiveProviderType } from '@/lib/shared/tts-provider-policy';
 import { resolveSegmentAudioUrls } from '@/lib/server/tts/segment-audio-urls';
 import { createRequestLogger } from '@/lib/server/logger';
@@ -71,7 +72,7 @@ function parseSettingsValue(value: unknown): TTSSegmentSettings | null {
   const nativeSpeed = Number.isFinite(Number(speedSource)) ? Number(speedSource) : 1;
   const instructionsSource = rec.ttsInstructions ?? rec.instructions;
   const ttsInstructions = typeof instructionsSource === 'string' ? instructionsSource : '';
-  const language = typeof rec.language === 'string' ? rec.language : 'en';
+  const language = typeof rec.language === 'string' ? normalizeLanguageTag(rec.language, 'en') : 'en';
 
   if (!providerRef || !providerType || !ttsModel || !voice) return null;
   return { providerRef, providerType, ttsModel, voice, nativeSpeed, ttsInstructions, language };
diff --git a/src/components/admin/AdminProvidersPanel.tsx b/src/components/admin/AdminProvidersPanel.tsx
index ef92a1d..4a2f202 100644
--- a/src/components/admin/AdminProvidersPanel.tsx
+++ b/src/components/admin/AdminProvidersPanel.tsx
@@ -4,7 +4,7 @@ import { useCallback, useEffect, useMemo, useState } from 'react';
 import { useMutation, useQuery, useQueryClient } from '@tanstack/react-query';
 import toast from 'react-hot-toast';
 import { DotsHorizontalIcon, PlusIcon } from '@/components/icons/Icons';
-import { providerSupportsCustomModel, resolveProviderModels, type TtsModelDefinition, type TtsProviderId } from '@/lib/shared/tts-provider-catalog';
+import { providerSupportsCustomModel, resolveProviderModels, type TtsModelDefinition, type TtsProviderId, type TtsProviderType } from '@/lib/shared/tts-provider-catalog';
 import { defaultBaseUrlForProviderType, defaultModelForProviderType, resolveTtsProviderModelPolicy } from '@/lib/shared/tts-provider-policy';
 import {
   Badge,
@@ -327,9 +327,9 @@ export function AdminProvidersPanel() {
       ? 'custom'
       : modelDefinitions[0]?.id ?? '';
   const selectedModelDefinition = modelDefinitions.find((model) => model.id === selectedModelId);
-  const modelSupportsInstructions = useCallback((model: string) => resolveTtsProviderModelPolicy({
+  const modelSupportsInstructions = useCallback((model: string, providerType: TtsProviderType = form.providerType) => resolveTtsProviderModelPolicy({
     providerRef: form.slug,
-    providerType: form.providerType,
+    providerType,
     model,
   }).supportsInstructions, [form.slug, form.providerType]);
   const baseUrlPlaceholder = form.providerType === 'custom-openai'
@@ -425,7 +425,7 @@ export function AdminProvidersPanel() {
                     providerType: opt.value,
                     baseUrl: opt.value === 'custom-openai' ? form.baseUrl : '',
                     defaultModel: nextModel,
-                    defaultInstructions: modelSupportsInstructions(nextModel) ? form.defaultInstructions : '',
+                    defaultInstructions: modelSupportsInstructions(nextModel, opt.value) ? form.defaultInstructions : '',
                   });
                   setCustomModelInput('');
                 }}
diff --git a/src/components/reader/SegmentsSidebar.tsx b/src/components/reader/SegmentsSidebar.tsx
index 835fe98..33d2434 100644
--- a/src/components/reader/SegmentsSidebar.tsx
+++ b/src/components/reader/SegmentsSidebar.tsx
@@ -188,6 +188,7 @@ export function SegmentsSidebar({ isOpen, setIsOpen, documentId, epubBookRef }:
     activeReaderType,
     clearSegmentCaches,
     resolvedLanguage,
+    setDocumentLanguage,
   } = useTTS();
   const {
     providerRef,
@@ -421,7 +422,8 @@ export function SegmentsSidebar({ isOpen, setIsOpen, documentId, epubBookRef }:
       updateConfigKey('voiceSpeed', Number.isFinite(Number(settings.nativeSpeed)) ? Number(settings.nativeSpeed) : 1),
       updateConfigKey('ttsInstructions', settings.ttsInstructions || ''),
     ]);
-  }, [updateConfigKey]);
+    if (settings.language) setDocumentLanguage(settings.language);
+  }, [updateConfigKey, setDocumentLanguage]);
 
   const handleRefresh = useCallback(() => {
     didAutoScrollOnOpenRef.current = false;
diff --git a/src/hooks/useDocumentLanguage.ts b/src/hooks/useDocumentLanguage.ts
index f8474ab..c8ca980 100644
--- a/src/hooks/useDocumentLanguage.ts
+++ b/src/hooks/useDocumentLanguage.ts
@@ -31,19 +31,22 @@ export function useDocumentLanguage(documentId: string | undefined): {
 
   const updateLanguage = useCallback(async (language: string): Promise<void> => {
     if (!documentId) return;
-    const next = mergeDocumentSettings(DEFAULT_DOCUMENT_SETTINGS, {
-      ...settings,
-      schemaVersion: 1,
-      language,
+    let next = DEFAULT_DOCUMENT_SETTINGS;
+    setSettings((prev) => {
+      next = mergeDocumentSettings(DEFAULT_DOCUMENT_SETTINGS, {
+        ...prev,
+        schemaVersion: 1,
+        language,
+      });
+      return next;
     });
-    setSettings(next);
     try {
       const response = await putDocumentSettings(documentId, next);
       setSettings(mergeDocumentSettings(DEFAULT_DOCUMENT_SETTINGS, response.settings));
     } catch (error) {
       console.warn('Failed to persist document language:', error);
     }
-  }, [documentId, settings]);
+  }, [documentId]);
 
   return {
     language: settings.language ?? 'auto',
diff --git a/src/lib/server/tts/generate.ts b/src/lib/server/tts/generate.ts
index c235e88..cfca3d3 100644
--- a/src/lib/server/tts/generate.ts
+++ b/src/lib/server/tts/generate.ts
@@ -644,7 +644,7 @@ async function runProviderRequest(
   }
 
   if (request.provider !== 'openai' && request.language) {
-    const supportKey = `${request.baseUrl || ''}|${request.model as string}`;
+    const supportKey = `${request.provider}|${request.baseUrl || ''}|${request.model as string}|${request.language}`;
     if (!openAiCompatibleLanguageUnsupported.has(supportKey)) {
       try {
         return await fetchTTSBufferWithRetry(
diff --git a/src/lib/server/tts/voice-resolution.ts b/src/lib/server/tts/voice-resolution.ts
index 1fd9420..7d80a13 100644
--- a/src/lib/server/tts/voice-resolution.ts
+++ b/src/lib/server/tts/voice-resolution.ts
@@ -311,7 +311,7 @@ export async function resolveReplicateLanguageInput({
   model,
   apiKey = '',
 }: ResolveVoicesOptions): Promise<ReplicateLanguageInput | null> {
-  if (provider !== 'replicate' || !apiKey) return null;
+  if (provider !== 'replicate' || REPLICATE_BUILT_IN_MODELS.has(model) || !apiKey) return null;
 
   const cached = replicateLanguageInputCache.get(model);
   if (cached) return cached;
diff --git a/tests/files/multilingual-sample.txt b/tests/files/multilingual-sample.txt
index fb517aa..5f9436c 100644
--- a/tests/files/multilingual-sample.txt
+++ b/tests/files/multilingual-sample.txt
@@ -2,7 +2,7 @@ English
 OpenReader should split this sentence correctly. This is the second sentence.
 
 French
-OpenReader doit correctement segmenter cette phrase. Voici la deuxieme phrase.
+OpenReader doit correctement segmenter cette phrase. Voici la deuxième phrase.
 
 Spanish
 OpenReader debe segmentar correctamente esta frase. Esta es la segunda frase.
diff --git a/tests/unit/audiobook-settings.vitest.spec.ts b/tests/unit/audiobook-settings.vitest.spec.ts
index 9cd68fe..4417d01 100644
--- a/tests/unit/audiobook-settings.vitest.spec.ts
+++ b/tests/unit/audiobook-settings.vitest.spec.ts
@@ -95,6 +95,7 @@ describe('coerceAudiobookGenerationSettings', () => {
         postSpeed: 1,
         format: 'mp3',
         ttsInstructions: '',
+        language: 'en',
       },
       restrictUserApiKeys: true,
       fallbackProviderRef: 'shared-openai',
@@ -113,5 +114,6 @@ describe('coerceAudiobookGenerationSettings', () => {
     expect(settings.providerType).toBe('openai');
     expect(settings.ttsModel).toBe('gpt-4o-mini-tts');
     expect(settings.ttsInstructions).toBe('Speak with warmth.');
+    expect(settings.language).toBe('en');
   });
 });