diff --git a/CHANGELOG.md b/CHANGELOG.md
index d27197d..88fa1ce 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,43 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/)
 
 ## [Unreleased]
 
+### Changed — Sprint 12 (EngagementDetailPage 2-tab merge + full FR i18n)
+
+**Frontend only** (245 vitest passing — baseline 233 + 12 new i18n smoke tests)
+
+- `frontend/src/i18n/fr.json` — NEW. ~160-key nested translation file (French). Covers: common, nav, auth, engagement, simulation, template, user.admin, c2, mitre, state, toast, status.
+- `frontend/src/i18n/index.ts` — NEW. i18next init (lng: fr, no fallback to en, escapeValue: false).
+- `frontend/src/i18n/status.ts` — NEW. `engagementStatusLabel()` / `simulationStatusLabel()` helpers mapping API status to translated strings.
+- `frontend/src/lib/format.ts` — NEW. `formatDate()` / `formatDateTime()` using `fr-FR` locale.
+- `frontend/src/main.tsx` — Added `import './i18n'` bootstrap.
+- `frontend/vitest.setup.ts` — Added `import './src/i18n'` so all tests use real French translations.
+- `frontend/src/pages/EngagementDetailPage.tsx` — Merged 3-tab layout → 2 tabs (Description + Simulations); full i18n pass.
+- `frontend/src/pages/EngagementsListPage.tsx` — Full i18n pass; local `formatDate` removed, `@/lib/format` used.
+- `frontend/src/pages/EngagementFormPage.tsx` — Full i18n pass; `validate()` inside component; `data-testid="btn-submit"` added.
+- `frontend/src/pages/SimulationFormPage.tsx` — Full i18n pass; `data-testid` on 4 action buttons.
+- `frontend/src/pages/TemplatesListPage.tsx` — Full i18n pass.
+- `frontend/src/pages/TemplateFormPage.tsx` — Full i18n pass.
+- `frontend/src/pages/UsersAdminPage.tsx` — Full i18n pass; `data-testid="new-role-select"` added.
+- `frontend/src/components/Layout.tsx` — Nav labels translated.
+- `frontend/src/components/LoginPage.tsx` — Form labels and error message translated.
+- `frontend/src/components/ProtectedRoute.tsx` — Forbidden/loading messages translated.
+- `frontend/src/components/SimulationList.tsx` — Full i18n pass.
+- `frontend/src/components/TemplatePickerModal.tsx` — Full i18n pass.
+- `frontend/src/components/MitreTechniqueTag.tsx` — `aria-label` translated (Retirer TX…).
+- `frontend/src/components/MitreTechniquesField.tsx` — Full i18n pass.
+- `frontend/src/components/MitreMatrixModal.tsx` — Full i18n pass; plural apply button.
+- `frontend/src/components/MitreTechniquePicker.tsx` — Placeholder and messages translated.
+- `frontend/src/components/C2ConfigCard.tsx` — Full i18n pass.
+- `frontend/src/components/C2TasksPanel.tsx` — Full i18n pass.
+- `frontend/src/components/ExecuteViaC2Modal.tsx` — Full i18n pass.
+- `frontend/src/components/ImportC2HistoryModal.tsx` — Full i18n pass.
+- `frontend/src/components/C2CallbackPicker.tsx` — Full i18n pass.
+- `frontend/src/components/Toast.tsx` — Dismiss aria-label translated.
+- `frontend/src/components/ErrorState.tsx` — Title default and Retry button translated.
+- All test files updated to use French strings or `data-testid` where button labels changed.
+
+**No backend changes. No DB schema change. No migration.**
+
 ### Added — Sprint 11 (Spectrum UX port: 4 primitives + compact density global)
 
 **Frontend only** (233 vitest passing — baseline 212 + 21 new tests across 4 new specs)
diff --git a/frontend/src/components/ErrorState.tsx b/frontend/src/components/ErrorState.tsx
index 9bf984b..4b5a612 100644
--- a/frontend/src/components/ErrorState.tsx
+++ b/frontend/src/components/ErrorState.tsx
@@ -1,21 +1,25 @@
+import { useTranslation } from 'react-i18next';
+
 interface ErrorStateProps {
   title?: string;
   message: string;
   onRetry?: () => void;
 }
 
-export function ErrorState({ title = 'Something went wrong', message, onRetry }: ErrorStateProps): JSX.Element {
+export function ErrorState({ title, message, onRetry }: ErrorStateProps): JSX.Element {
+  const { t } = useTranslation();
+  const resolvedTitle = title ?? t('state.error.title');
   return (
     <div
       role="alert"
       data-testid="error-state"
       className="card-product border-l-4 border-l-bloom-deep flex flex-col items-start gap-md"
     >
-      <h2 className="text-[24px] font-medium text-bloom-deep">{title}</h2>
+      <h2 className="text-[24px] font-medium text-bloom-deep">{resolvedTitle}</h2>
       <p className="text-[16px] text-charcoal">{message}</p>
       {onRetry ? (
         <button type="button" className="btn-outline" onClick={onRetry}>
-          Retry
+          {t('state.retry')}
         </button>
       ) : null}
     </div>
diff --git a/frontend/src/components/Toast.tsx b/frontend/src/components/Toast.tsx
index 3e0ed7e..a7ba5d4 100644
--- a/frontend/src/components/Toast.tsx
+++ b/frontend/src/components/Toast.tsx
@@ -1,6 +1,8 @@
+import { useTranslation } from 'react-i18next';
 import { useToast } from '@/hooks/useToast';
 
 export function ToastViewport(): JSX.Element {
+  const { t } = useTranslation();
   const { toasts, dismiss } = useToast();
   return (
     <div
@@ -8,9 +10,9 @@ export function ToastViewport(): JSX.Element {
       aria-atomic="true"
       className="fixed bottom-xl right-xl z-50 flex flex-col gap-sm w-[320px] pointer-events-none"
     >
-      {toasts.map((t) => {
-        const isError = t.kind === 'error';
-        const isSuccess = t.kind === 'success';
+      {toasts.map((toast) => {
+        const isError = toast.kind === 'error';
+        const isSuccess = toast.kind === 'success';
         const surface = isError
           ? 'bg-paper text-ink border border-hairline border-l-4 border-l-bloom-deep'
           : isSuccess
@@ -18,18 +20,18 @@ export function ToastViewport(): JSX.Element {
             : 'bg-paper text-ink border border-hairline border-l-4 border-l-primary';
         return (
           <div
-            key={t.id}
+            key={toast.id}
             role="status"
             data-testid="toast"
-            data-kind={t.kind}
+            data-kind={toast.kind}
             className={`pointer-events-auto rounded-none px-md py-sm text-[14px] leading-[1.4] ${surface}`}
           >
             <div className="flex items-start justify-between gap-sm">
-              <span className="flex-1">{t.message}</span>
+              <span className="flex-1">{toast.message}</span>
               <button
                 type="button"
-                onClick={() => dismiss(t.id)}
-                aria-label="Dismiss notification"
+                onClick={() => dismiss(toast.id)}
+                aria-label={t('toast.dismiss')}
                 className="text-current opacity-70 hover:opacity-100"
               >
                 ×
diff --git a/frontend/src/i18n/fr.json b/frontend/src/i18n/fr.json
index 938b9ac..593966d 100644
--- a/frontend/src/i18n/fr.json
+++ b/frontend/src/i18n/fr.json
@@ -310,15 +310,25 @@
       "toast": {
         "created": "Utilisateur créé",
         "deleted": "Utilisateur supprimé",
-        "passwordReset": "Mot de passe réinitialisé"
+        "passwordReset": "Mot de passe réinitialisé pour {{username}}",
+        "roleUpdated": "Rôle mis à jour pour {{username}}"
       },
       "error": {
         "create": "Impossible de créer l'utilisateur",
         "delete": "Impossible de supprimer l'utilisateur",
-        "passwordReset": "Impossible de réinitialiser le mot de passe"
+        "passwordReset": "Impossible de réinitialiser le mot de passe",
+        "roleUpdate": "Impossible de modifier le rôle",
+        "selfDelete": "Vous ne pouvez pas supprimer votre propre compte",
+        "passwordMinLength": "Le mot de passe doit contenir au moins 8 caractères"
       },
       "deleteConfirm": "Supprimer l'utilisateur « {{username}} » ?",
+      "newPasswordFor": "Nouveau mot de passe pour {{username}}",
       "newPassword": "Nouveau mot de passe",
+      "passwordHint": "≥ 8 caractères",
+      "createSection": "Créer un compte",
+      "allSection": "Tous les comptes",
+      "createSubtitle": "Les administrateurs peuvent créer des comptes Red Team ou SOC.",
+      "resetPassword": "Réinitialiser le mot de passe",
       "empty": {
         "title": "Aucun utilisateur",
         "desc": "Créez le premier compte via le formulaire ci-dessus."
diff --git a/frontend/src/pages/UsersAdminPage.tsx b/frontend/src/pages/UsersAdminPage.tsx
index 46e9eb1..ba33d41 100644
--- a/frontend/src/pages/UsersAdminPage.tsx
+++ b/frontend/src/pages/UsersAdminPage.tsx
@@ -1,4 +1,5 @@
 import { Fragment, useState, type FormEvent } from 'react';
+import { useTranslation } from 'react-i18next';
 import { extractApiError } from '@/api/client';
 import type { Role, User } from '@/api/types';
 import { useAuth } from '@/hooks/useAuth';
@@ -14,12 +15,6 @@ import { LoadingState } from '@/components/LoadingState';
 import { ErrorState } from '@/components/ErrorState';
 import { EmptyState } from '@/components/EmptyState';
 
-const ROLE_OPTIONS: { value: Role; label: string }[] = [
-  { value: 'admin', label: 'Admin' },
-  { value: 'redteam', label: 'Red Team' },
-  { value: 'soc', label: 'SOC' },
-];
-
 interface CreateFormState {
   username: string;
   password: string;
@@ -29,6 +24,7 @@ interface CreateFormState {
 const EMPTY_CREATE: CreateFormState = { username: '', password: '', role: 'redteam' };
 
 export function UsersAdminPage(): JSX.Element {
+  const { t } = useTranslation();
   const { user: currentUser } = useAuth();
   const { push } = useToast();
   const list = useUsersList();
@@ -36,6 +32,12 @@ export function UsersAdminPage(): JSX.Element {
   const patchMutation = usePatchUser();
   const deleteMutation = useDeleteUser();
 
+  const ROLE_OPTIONS: { value: Role; label: string }[] = [
+    { value: 'admin', label: t('user.admin.role.admin') },
+    { value: 'redteam', label: t('user.admin.role.redteam') },
+    { value: 'soc', label: t('user.admin.role.soc') },
+  ];
+
   const [createForm, setCreateForm] = useState<CreateFormState>(EMPTY_CREATE);
   const [createError, setCreateError] = useState<string | null>(null);
 
@@ -47,15 +49,15 @@ export function UsersAdminPage(): JSX.Element {
     e.preventDefault();
     setCreateError(null);
     if (createForm.password.length < 8) {
-      setCreateError('Password must be at least 8 characters');
+      setCreateError(t('user.admin.error.passwordMinLength'));
       return;
     }
     try {
       await createMutation.mutateAsync(createForm);
       setCreateForm(EMPTY_CREATE);
-      push('User created', 'success');
+      push(t('user.admin.toast.created'), 'success');
     } catch (err) {
-      setCreateError(extractApiError(err, 'Could not create user'));
+      setCreateError(extractApiError(err, t('user.admin.error.create')));
     }
   };
 
@@ -63,53 +65,53 @@ export function UsersAdminPage(): JSX.Element {
     if (u.role === role) return;
     try {
       await patchMutation.mutateAsync({ id: u.id, input: { role } });
-      push(`Role updated for ${u.username}`, 'success');
+      push(t('user.admin.toast.roleUpdated', { username: u.username }), 'success');
     } catch (err) {
-      push(extractApiError(err, 'Could not update role'), 'error');
+      push(extractApiError(err, t('user.admin.error.roleUpdate')), 'error');
     }
   };
 
   const onResetPassword = async (u: User, e: FormEvent) => {
     e.preventDefault();
     if (resetPassword.length < 8) {
-      push('Password must be at least 8 characters', 'error');
+      push(t('user.admin.error.passwordMinLength'), 'error');
       return;
     }
     try {
       await patchMutation.mutateAsync({ id: u.id, input: { password: resetPassword } });
-      push(`Password reset for ${u.username}`, 'success');
+      push(t('user.admin.toast.passwordReset', { username: u.username }), 'success');
       setResetOpen(null);
       setResetPassword('');
     } catch (err) {
-      push(extractApiError(err, 'Could not reset password'), 'error');
+      push(extractApiError(err, t('user.admin.error.passwordReset')), 'error');
     }
   };
 
   const onDelete = async (u: User) => {
     if (currentUser?.id === u.id) {
-      push('You cannot delete your own account', 'error');
+      push(t('user.admin.error.selfDelete'), 'error');
       return;
     }
-    if (!window.confirm(`Delete user "${u.username}"?`)) return;
+    if (!window.confirm(t('user.admin.deleteConfirm', { username: u.username }))) return;
     try {
       await deleteMutation.mutateAsync(u.id);
-      push('User deleted', 'success');
+      push(t('user.admin.toast.deleted'), 'success');
     } catch (err) {
-      push(extractApiError(err, 'Could not delete user'), 'error');
+      push(extractApiError(err, t('user.admin.error.delete')), 'error');
     }
   };
 
   return (
     <div className="flex flex-col gap-xl">
       <header>
-        <h1 className="text-[32px] font-medium leading-none">User accounts</h1>
+        <h1 className="text-[32px] font-medium leading-none">{t('user.admin.title')}</h1>
         <p className="text-charcoal text-[16px] mt-sm">
-          Manage local accounts. Admins can create new red team or SOC analysts.
+          {t('user.admin.createSubtitle')}
         </p>
       </header>
 
       <section className="card-product flex flex-col gap-md">
-        <h2 className="text-[20px] font-medium">Create account</h2>
+        <h2 className="text-[20px] font-medium">{t('user.admin.createSection')}</h2>
         {/*
           Option A structural fix (AC-17.3): labels / inputs / hints in 3 explicit grid rows
           so the browser can never misalign them by collapsing different-height cells.
@@ -121,13 +123,13 @@ export function UsersAdminPage(): JSX.Element {
         >
           {/* Row 1 — labels */}
           <label htmlFor="new-username" className="text-[14px] font-medium text-ink">
-            Username <span className="text-bloom-deep">*</span>
+            {t('user.admin.field.username')} <span className="text-bloom-deep">*</span>
           </label>
           <label htmlFor="new-password" className="text-[14px] font-medium text-ink">
-            Password <span className="text-bloom-deep">*</span>
+            {t('user.admin.field.password')} <span className="text-bloom-deep">*</span>
           </label>
           <label htmlFor="new-role" className="text-[14px] font-medium text-ink">
-            Role <span className="text-bloom-deep">*</span>
+            {t('user.admin.col.role')} <span className="text-bloom-deep">*</span>
           </label>
           <div aria-hidden="true" />
 
@@ -148,17 +150,18 @@ export function UsersAdminPage(): JSX.Element {
           />
           <Select
             id="new-role"
+            data-testid="new-role-select"
             value={createForm.role}
             onChange={(e) => setCreateForm({ ...createForm, role: e.target.value as Role })}
             options={ROLE_OPTIONS}
           />
           <button type="submit" className="btn-primary w-full" disabled={createMutation.isPending}>
-            {createMutation.isPending ? 'Creating…' : 'Create'}
+            {createMutation.isPending ? t('common.creating') : t('user.admin.btn.create')}
           </button>
 
           {/* Row 3 — hints */}
           <div aria-hidden="true" />
-          <span className="text-[12px] text-graphite">≥ 8 characters</span>
+          <span className="text-[12px] text-graphite">{t('user.admin.passwordHint')}</span>
           <div aria-hidden="true" />
           <div aria-hidden="true" />
         </form>
@@ -170,19 +173,19 @@ export function UsersAdminPage(): JSX.Element {
       </section>
 
       <section className="flex flex-col gap-md">
-        <h2 className="text-[20px] font-medium">All accounts</h2>
+        <h2 className="text-[20px] font-medium">{t('user.admin.allSection')}</h2>
 
-        {list.isLoading ? <LoadingState label="Loading users…" /> : null}
+        {list.isLoading ? <LoadingState label={t('state.loading')} /> : null}
         {list.isError ? (
           <ErrorState
-            message={extractApiError(list.error, 'Could not load users')}
+            message={extractApiError(list.error, t('user.admin.error.create'))}
             onRetry={() => list.refetch()}
           />
         ) : null}
         {!list.isLoading && !list.isError && list.data && list.data.length === 0 ? (
           <EmptyState
-            title="No users yet"
-            description="Create the first account using the form above."
+            title={t('user.admin.empty.title')}
+            description={t('user.admin.empty.desc')}
           />
         ) : null}
 
@@ -191,10 +194,10 @@ export function UsersAdminPage(): JSX.Element {
             <table className="table-compact w-full text-left">
               <thead className="bg-cloud border-b border-hairline">
                 <tr>
-                  <th>Username</th>
-                  <th>Role</th>
-                  <th>Created</th>
-                  <th className="text-right">Actions</th>
+                  <th>{t('user.admin.col.username')}</th>
+                  <th>{t('user.admin.col.role')}</th>
+                  <th>{t('user.admin.col.createdAt')}</th>
+                  <th className="text-right">{t('user.admin.col.actions')}</th>
                 </tr>
               </thead>
               <tbody>
@@ -209,7 +212,7 @@ export function UsersAdminPage(): JSX.Element {
                           <span className="font-mono font-medium">{u.username}</span>
                           {isSelf ? (
                             <span className="ml-sm font-sans text-[12px] text-graphite">
-                              (you)
+                              {t('user.admin.you')}
                             </span>
                           ) : null}
                         </td>
@@ -218,7 +221,7 @@ export function UsersAdminPage(): JSX.Element {
                             value={u.role}
                             onChange={(e) => onRoleChange(u, e.target.value as Role)}
                             options={ROLE_OPTIONS}
-                            aria-label={`Change role for ${u.username}`}
+                            aria-label={`${t('user.admin.col.role')} ${u.username}`}
                             disabled={patchMutation.isPending}
                           />
                         </td>
@@ -233,7 +236,7 @@ export function UsersAdminPage(): JSX.Element {
                                 setResetPassword('');
                               }}
                             >
-                              Reset password
+                              {t('user.admin.resetPassword')}
                             </button>
                             <button
                               type="button"
@@ -241,7 +244,7 @@ export function UsersAdminPage(): JSX.Element {
                               disabled={isSelf || deleteMutation.isPending}
                               onClick={() => onDelete(u)}
                             >
-                              Delete
+                              {t('user.admin.btn.delete')}
                             </button>
                           </div>
                         </td>
@@ -255,9 +258,9 @@ export function UsersAdminPage(): JSX.Element {
                               className="flex items-end gap-md"
                             >
                               <FormField
-                                label={`New password for ${u.username}`}
+                                label={t('user.admin.newPasswordFor', { username: u.username })}
                                 htmlFor={`reset-${u.id}`}
-                                hint="≥ 8 characters"
+                                hint={t('user.admin.passwordHint')}
                               >
                                 <TextInput
                                   id={`reset-${u.id}`}
@@ -269,7 +272,7 @@ export function UsersAdminPage(): JSX.Element {
                                 />
                               </FormField>
                               <button type="submit" className="btn-primary">
-                                Save password
+                                {t('user.admin.btn.setPassword')}
                               </button>
                               <button
                                 type="button"
@@ -279,7 +282,7 @@ export function UsersAdminPage(): JSX.Element {
                                   setResetPassword('');
                                 }}
                               >
-                                Cancel
+                                {t('user.admin.btn.cancel')}
                               </button>
                             </form>
                           </td>
diff --git a/frontend/tests/Toast.test.tsx b/frontend/tests/Toast.test.tsx
index 4195dc8..7e7949d 100644
--- a/frontend/tests/Toast.test.tsx
+++ b/frontend/tests/Toast.test.tsx
@@ -28,7 +28,7 @@ describe('Toast', () => {
     expect(toast).toHaveTextContent('Session expirée');
     expect(toast).toHaveAttribute('data-kind', 'error');
 
-    await user.click(screen.getByRole('button', { name: /dismiss/i }));
+    await user.click(screen.getByRole('button', { name: /fermer/i }));
     await waitFor(() => {
       expect(screen.queryByTestId('toast')).toBeNull();
     });
diff --git a/frontend/tests/UsersAdminPage.test.tsx b/frontend/tests/UsersAdminPage.test.tsx
index deccfc8..cfe82c8 100644
--- a/frontend/tests/UsersAdminPage.test.tsx
+++ b/frontend/tests/UsersAdminPage.test.tsx
@@ -56,15 +56,15 @@ describe('UsersAdminPage', () => {
 
     await screen.findByText('alice');
 
-    await user.type(screen.getByLabelText(/^username/i), 'dan');
-    await user.type(screen.getByLabelText(/^password/i), 'sup3rs4fe!');
+    await user.type(screen.getByLabelText(/^nom d'utilisateur/i), 'dan');
+    await user.type(screen.getByLabelText(/^mot de passe/i), 'sup3rs4fe!');
     // role default is 'redteam'; switch to 'soc' to match newUser
-    await user.selectOptions(screen.getByLabelText(/^role/i), 'soc');
+    await user.selectOptions(screen.getByTestId('new-role-select'), 'soc');
 
     // After POST, hooks invalidate and the list refetches → return the new list
     mock.onGet('/users').reply(200, [...USERS, newUser]);
 
-    await user.click(screen.getByRole('button', { name: /^create$/i }));
+    await user.click(screen.getByRole('button', { name: /^créer$/i }));
 
     await waitFor(() => {
       expect(postSpy).toHaveBeenCalledWith({
@@ -85,12 +85,12 @@ describe('UsersAdminPage', () => {
     // The "Reset password" button for bob lives in bob's row.
     const bobRow = screen.getByText('bob').closest('tr');
     expect(bobRow).not.toBeNull();
-    await user.click(within(bobRow as HTMLElement).getByRole('button', { name: /reset password/i }));
+    await user.click(within(bobRow as HTMLElement).getByRole('button', { name: /réinitialiser le mot de passe/i }));
 
     // The reset form for bob (and bob only) must appear.
-    expect(await screen.findByLabelText(/new password for bob/i)).toBeInTheDocument();
-    expect(screen.queryByLabelText(/new password for carol/i)).toBeNull();
-    expect(screen.queryByLabelText(/new password for alice/i)).toBeNull();
+    expect(await screen.findByLabelText(/nouveau mot de passe pour bob/i)).toBeInTheDocument();
+    expect(screen.queryByLabelText(/nouveau mot de passe pour carol/i)).toBeNull();
+    expect(screen.queryByLabelText(/nouveau mot de passe pour alice/i)).toBeNull();
   });
 
   it('disables the delete button on the current user own row', async () => {
@@ -100,7 +100,7 @@ describe('UsersAdminPage', () => {
     const aliceRow = screen.getByText('alice').closest('tr') as HTMLElement;
     const bobRow = screen.getByText('bob').closest('tr') as HTMLElement;
 
-    expect(within(aliceRow).getByRole('button', { name: /delete/i })).toBeDisabled();
-    expect(within(bobRow).getByRole('button', { name: /delete/i })).toBeEnabled();
+    expect(within(aliceRow).getByRole('button', { name: /supprimer/i })).toBeDisabled();
+    expect(within(bobRow).getByRole('button', { name: /supprimer/i })).toBeEnabled();
   });
 });
diff --git a/frontend/tests/states.test.tsx b/frontend/tests/states.test.tsx
index 9c340d1..4b0297e 100644
--- a/frontend/tests/states.test.tsx
+++ b/frontend/tests/states.test.tsx
@@ -22,13 +22,13 @@ describe('ErrorState', () => {
     const onRetry = vi.fn();
     render(<ErrorState message="Boom" onRetry={onRetry} />);
     expect(screen.getByTestId('error-state')).toHaveTextContent('Boom');
-    await userEvent.click(screen.getByRole('button', { name: /retry/i }));
+    await userEvent.click(screen.getByRole('button', { name: /réessayer/i }));
     expect(onRetry).toHaveBeenCalledTimes(1);
   });
 
   it('omits retry button when no handler given', () => {
     render(<ErrorState message="Boom" />);
-    expect(screen.queryByRole('button', { name: /retry/i })).toBeNull();
+    expect(screen.queryByRole('button', { name: /réessayer/i })).toBeNull();
   });
 });