diff --git a/src/app/Livewire/DocumentEditor.php b/src/app/Livewire/DocumentEditor.php
index cc8ccfc..f6abc43 100644
--- a/src/app/Livewire/DocumentEditor.php
+++ b/src/app/Livewire/DocumentEditor.php
@@ -2,30 +2,44 @@
 
 namespace App\Livewire;
 
+use App\Http\Middleware\SetLocale;
 use App\Models\Document;
 use App\Services\DocumentService;
-use Livewire\Component;
+use Illuminate\Support\Facades\App;
 use Illuminate\Support\Facades\Auth;
+use Livewire\Component;
 
 class DocumentEditor extends Component
 {
     public ?Document $document = null;
-    public $title = '';
-    public $content = '';
-    public $directory = '';
-    public $isEditMode = false;
+    public string $title = '';
+    public string $content = '';
+    public string $editingLocale = '';
+    public bool $isEditMode = false;
+    public bool $isNewLocale = false;
+    public array $availableLocales = [];
 
-    public function mount(?Document $document = null)
+    public function mount(?Document $document = null, ?string $locale = null)
     {
         if ($document) {
             $this->authorize('update', $document);
-
-            $this->document = $document;
-            $this->title = $document->title;
-            $this->content = $document->content;
-            $this->directory = $document->directory;
+            $this->document = $document->load('translations');
             $this->isEditMode = true;
+            $this->availableLocales = $document->availableLocales();
+            $this->editingLocale = $locale ?: ($document->default_locale ?? App::getLocale());
+
+            $translation = $document->translations->firstWhere('locale', $this->editingLocale);
+            if ($translation) {
+                $this->title = $translation->title;
+                $this->content = $translation->content;
+                $this->isNewLocale = false;
+            } else {
+                $this->title = '';
+                $this->content = '';
+                $this->isNewLocale = true;
+            }
         } else {
+            $this->editingLocale = App::getLocale();
             $titleParam = request()->query('title');
             if ($titleParam) {
                 $this->title = $titleParam;
@@ -35,53 +49,96 @@ public function mount(?Document $document = null)
 
     public function save(DocumentService $documentService)
     {
-        $this->validate([
+        $validated = $this->validate([
             'title' => 'required|string|max:255',
             'content' => 'required|string',
+            'editingLocale' => ['required', 'string', 'in:' . implode(',', array_keys(SetLocale::SUPPORTED_LOCALES))],
         ]);
 
         try {
             if ($this->isEditMode && $this->document) {
                 $this->authorize('update', $this->document);
 
-                $this->document = $documentService->updateDocument(
-                    $this->document,
-                    $this->title,
-                    $this->content,
-                    Auth::id()
-                );
+                if ($this->isNewLocale) {
+                    $documentService->addTranslation(
+                        $this->document,
+                        $this->editingLocale,
+                        $this->title,
+                        $this->content,
+                        Auth::id(),
+                    );
+                    $this->document->refresh()->load('translations');
+                } else {
+                    $this->document = $documentService->updateDocument(
+                        $this->document,
+                        $this->title,
+                        $this->content,
+                        Auth::id(),
+                        $this->editingLocale,
+                    );
+                }
 
-                session()->flash('message', 'Document updated successfully!');
+                session()->flash('message', __('messages.documents.update_success'));
                 return $this->redirect(route('documents.show', $this->document));
             } else {
                 $this->document = $documentService->createDocument(
                     $this->title,
                     $this->content,
                     Auth::id(),
-                    $this->directory ?: null
+                    $this->editingLocale,
                 );
-
-                session()->flash('message', 'Document created successfully!');
+                session()->flash('message', __('messages.documents.create_success'));
                 return $this->redirect(route('documents.show', $this->document));
             }
+        } catch (\InvalidArgumentException $e) {
+            session()->flash('error', $e->getMessage());
         } catch (\Exception $e) {
             session()->flash('error', 'Error saving document: ' . $e->getMessage());
         }
     }
 
+    public function deleteTranslation(DocumentService $documentService)
+    {
+        if (!$this->isEditMode || !$this->document || $this->isNewLocale) {
+            return;
+        }
+        $this->authorize('update', $this->document);
+
+        try {
+            $documentService->deleteTranslation($this->document, $this->editingLocale);
+            session()->flash('message', __('messages.documents.translation_deleted'));
+            return $this->redirect(route('documents.show', $this->document));
+        } catch (\InvalidArgumentException $e) {
+            session()->flash('error', $e->getMessage());
+        }
+    }
+
+    public function setAsDefault(DocumentService $documentService)
+    {
+        if (!$this->isEditMode || !$this->document) {
+            return;
+        }
+        $this->authorize('update', $this->document);
+
+        try {
+            $this->document = $documentService->setDefaultLocale($this->document, $this->editingLocale);
+            session()->flash('message', __('messages.documents.update_success'));
+            return $this->redirect(route('documents.show', $this->document));
+        } catch (\InvalidArgumentException $e) {
+            session()->flash('error', $e->getMessage());
+        }
+    }
+
     public function delete(DocumentService $documentService)
     {
         if (!$this->isEditMode || !$this->document) {
             return;
         }
-
         $this->authorize('delete', $this->document);
 
         try {
             $documentService->deleteDocument($this->document);
-            session()->flash('message', 'Document deleted successfully!');
-
-            // Try to redirect to home document, or root if not found
+            session()->flash('message', __('messages.documents.delete_success'));
             $homeDocument = Document::where('slug', 'home')->first();
             if ($homeDocument) {
                 return redirect()->route('documents.show', $homeDocument);
@@ -96,7 +153,9 @@ public function render()
     {
         return view('livewire.document-editor')
             ->layout('layouts.knowledge-base', [
-                'title' => $this->isEditMode ? 'Edit: ' . $this->title : 'New Document'
+                'title' => $this->isEditMode
+                    ? __('messages.documents.edit_document') . ': ' . $this->title
+                    : __('messages.documents.new_document'),
             ]);
     }
 }
diff --git a/src/resources/views/livewire/document-editor.blade.php b/src/resources/views/livewire/document-editor.blade.php
index 5b1bbe9..f687fa1 100644
--- a/src/resources/views/livewire/document-editor.blade.php
+++ b/src/resources/views/livewire/document-editor.blade.php
@@ -56,6 +56,65 @@ class="inline-flex items-center justify-center px-3 sm:px-4 py-2 bg-indigo-600 b
         </div>
     </div>
 
+    @if($isEditMode && $document)
+        <div class="mb-4 border-b border-gray-200" role="tablist" aria-label="{{ __('messages.documents.translation_tabs_label') }}">
+            <nav class="-mb-px flex flex-wrap gap-x-2">
+                @php $allLocales = \App\Http\Middleware\SetLocale::SUPPORTED_LOCALES; @endphp
+                @foreach($availableLocales as $loc)
+                    @php $isActive = ($loc === $editingLocale); @endphp
+                    <a href="{{ route('documents.translations.edit', ['document' => $document, 'locale' => $loc]) }}"
+                       class="px-3 py-2 text-sm font-medium border-b-2 {{ $isActive ? 'border-indigo-500 text-indigo-600' : 'border-transparent text-gray-500 hover:text-gray-700 hover:border-gray-300' }}">
+                        {{ __('messages.locale_names.' . $loc, [], 'en') }}
+                        @if($loc === $document->default_locale)
+                            <span class="ml-1 text-xs text-gray-400">★</span>
+                        @endif
+                    </a>
+                @endforeach
+
+                @if($isNewLocale && $editingLocale)
+                    <span class="px-3 py-2 text-sm font-medium border-b-2 border-indigo-500 text-indigo-600">
+                        {{ __('messages.locale_names.' . $editingLocale, [], 'en') }}
+                        <span class="ml-1 text-xs text-gray-400">({{ __('messages.documents.new_document') }})</span>
+                    </span>
+                @endif
+
+                @php $missingLocales = array_diff(array_keys($allLocales), $availableLocales, $isNewLocale ? [$editingLocale] : []); @endphp
+                @if(!empty($missingLocales))
+                    <div x-data="{ open: false }" class="relative">
+                        <button type="button" @click="open = !open"
+                                class="px-3 py-2 text-sm font-medium border-b-2 border-transparent text-gray-500 hover:text-gray-700 hover:border-gray-300">
+                            + {{ __('messages.documents.add_translation') }}
+                        </button>
+                        <div x-show="open" @click.outside="open = false" x-cloak
+                             class="absolute right-0 z-10 mt-2 w-48 max-h-64 overflow-y-auto bg-white border border-gray-200 rounded-md shadow-lg">
+                            @foreach($missingLocales as $loc)
+                                <a href="{{ route('documents.translations.edit', ['document' => $document, 'locale' => $loc]) }}"
+                                   class="block px-3 py-2 text-sm text-gray-700 hover:bg-gray-50">
+                                    {{ $allLocales[$loc] }}
+                                </a>
+                            @endforeach
+                        </div>
+                    </div>
+                @endif
+            </nav>
+
+            @if($editingLocale !== $document->default_locale && !$isNewLocale)
+                <div class="mt-2 flex gap-2">
+                    <button wire:click="setAsDefault" type="button"
+                            class="px-2 py-1 text-xs text-indigo-600 border border-indigo-300 rounded hover:bg-indigo-50">
+                        {{ __('messages.documents.set_as_default') }}
+                    </button>
+                    <button wire:click="deleteTranslation"
+                            wire:confirm="{{ __('messages.documents.delete_confirm') }}"
+                            type="button"
+                            class="px-2 py-1 text-xs text-red-600 border border-red-300 rounded hover:bg-red-50">
+                        {{ __('messages.documents.delete_translation') ?? __('messages.documents.delete') }}
+                    </button>
+                </div>
+            @endif
+        </div>
+    @endif
+
     <!-- Form -->
     <form wire:submit.prevent="save" class="space-y-6">
         <!-- Title -->
diff --git a/src/tests/Feature/DocumentI18nTest.php b/src/tests/Feature/DocumentI18nTest.php
index 9404ee7..ec8a730 100644
--- a/src/tests/Feature/DocumentI18nTest.php
+++ b/src/tests/Feature/DocumentI18nTest.php
@@ -65,4 +65,41 @@ public function test_no_banner_when_translation_exists(): void
         $response->assertOk();
         $response->assertDontSeeText(__('messages.documents.fallback_notice', [], 'ja'));
     }
+
+    public function test_editor_loads_existing_translation_for_locale(): void
+    {
+        $owner = \App\Models\User::factory()->create();
+        $doc = Document::factory()->create(['default_locale' => 'en', 'created_by' => $owner->id, 'slug' => 'editor']);
+        $doc->translations()->where('locale', 'en')->update(['title' => 'Hello', 'content' => 'EN body']);
+        DocumentTranslation::factory()->create([
+            'document_id' => $doc->id,
+            'locale' => 'ja',
+            'title' => 'こんにちは',
+            'content' => 'JA body',
+        ]);
+
+        $response = $this->actingAs($owner)->get(route('documents.translations.edit', [
+            'document' => $doc,
+            'locale' => 'ja',
+        ]));
+
+        $response->assertOk();
+        $response->assertSee('こんにちは');
+        $response->assertSee('JA body');
+    }
+
+    public function test_editor_for_missing_locale_shows_empty_form_with_new_locale_state(): void
+    {
+        $owner = \App\Models\User::factory()->create();
+        $doc = Document::factory()->create(['default_locale' => 'en', 'created_by' => $owner->id, 'slug' => 'editor2']);
+
+        $response = $this->actingAs($owner)->get(route('documents.translations.edit', [
+            'document' => $doc,
+            'locale' => 'ja',
+        ]));
+
+        $response->assertOk();
+        // The blade should render a tab marked active for ja with empty inputs
+        $response->assertSeeText(__('messages.locale_names.ja', [], 'en'));
+    }
 }