diff --git a/packages/javascript/src/api/__tests__/executeEmbeddedSignInFlow.test.ts b/packages/javascript/src/api/__tests__/executeEmbeddedSignInFlow.test.ts
index edc6ca18..976b09e7 100644
--- a/packages/javascript/src/api/__tests__/executeEmbeddedSignInFlow.test.ts
+++ b/packages/javascript/src/api/__tests__/executeEmbeddedSignInFlow.test.ts
@@ -45,6 +45,16 @@ describe('executeEmbeddedSignInFlow', (): void => {
       expect(captureRequestBody()).toMatchObject({verbose: true});
     });
 
+    it('sends a top-level locale field through to the request body', async (): Promise<void> => {
+      await executeEmbeddedSignInFlow({
+        payload: {action: 'submit', executionId: 'exec-abc', inputs: {username: 'user'}, locale: 'es-AR'},
+        url: URL,
+      });
+
+      const body = captureRequestBody();
+      expect(body).toMatchObject({locale: 'es-AR', inputs: {username: 'user'}});
+    });
+
     it('injects verbose:true for a new flow start that also includes scopes', async (): Promise<void> => {
       await executeEmbeddedSignInFlow({
         payload: {applicationId: 'app-1', flowType: 'AUTHENTICATION', scopes: ['openid', 'profile']},
diff --git a/packages/javascript/src/api/__tests__/executeEmbeddedSignUpFlow.test.ts b/packages/javascript/src/api/__tests__/executeEmbeddedSignUpFlow.test.ts
index 7258a14f..84fbdac4 100644
--- a/packages/javascript/src/api/__tests__/executeEmbeddedSignUpFlow.test.ts
+++ b/packages/javascript/src/api/__tests__/executeEmbeddedSignUpFlow.test.ts
@@ -38,6 +38,16 @@ describe('executeEmbeddedSignUpFlow', (): void => {
       expect(captureRequestBody()).toMatchObject({verbose: true});
     });
 
+    it('sends a top-level locale field through to the request body', async (): Promise<void> => {
+      await executeEmbeddedSignUpFlow({
+        payload: {action: 'submit', executionId: 'exec-abc', inputs: {username: 'user'}, locale: 'es-AR'},
+        url: URL,
+      });
+
+      const body = captureRequestBody();
+      expect(body).toMatchObject({locale: 'es-AR', inputs: {username: 'user'}});
+    });
+
     it('injects verbose:true for a new flow start that also includes scopes', async (): Promise<void> => {
       await executeEmbeddedSignUpFlow({
         payload: {applicationId: 'app-1', flowType: 'REGISTRATION', scopes: ['openid', 'profile']},
diff --git a/packages/javascript/src/models/embedded-recovery-flow.ts b/packages/javascript/src/models/embedded-recovery-flow.ts
index 06019655..aca8019b 100644
--- a/packages/javascript/src/models/embedded-recovery-flow.ts
+++ b/packages/javascript/src/models/embedded-recovery-flow.ts
@@ -117,6 +117,7 @@ export interface EmbeddedRecoveryFlowRequest extends Partial<EmbeddedRecoveryFlo
   challengeToken?: string;
   executionId?: string;
   inputs?: Record<string, any>;
+  locale?: string;
 }
 
 /**
diff --git a/packages/javascript/src/models/embedded-signin-flow.ts b/packages/javascript/src/models/embedded-signin-flow.ts
index bff79add..41406f3b 100644
--- a/packages/javascript/src/models/embedded-signin-flow.ts
+++ b/packages/javascript/src/models/embedded-signin-flow.ts
@@ -359,4 +359,7 @@ export interface EmbeddedSignInFlowRequest extends Partial<EmbeddedSignInFlowIni
    * ```
    */
   inputs?: Record<string, any>;
+
+  /** Current UI language (BCP 47) so the flow localizes notifications; validated server-side. */
+  locale?: string;
 }
diff --git a/packages/javascript/src/models/embedded-signup-flow.ts b/packages/javascript/src/models/embedded-signup-flow.ts
index 6e26dde6..eb3bf247 100644
--- a/packages/javascript/src/models/embedded-signup-flow.ts
+++ b/packages/javascript/src/models/embedded-signup-flow.ts
@@ -211,6 +211,7 @@ export interface EmbeddedSignUpFlowRequest extends Partial<EmbeddedSignUpFlowIni
   challengeToken?: string;
   executionId?: string;
   inputs?: Record<string, any>;
+  locale?: string;
 }
 
 /**
diff --git a/packages/react/src/components/presentation/auth/AcceptInvite/AcceptInvite.tsx b/packages/react/src/components/presentation/auth/AcceptInvite/AcceptInvite.tsx
index 3382d0d9..00e68621 100644
--- a/packages/react/src/components/presentation/auth/AcceptInvite/AcceptInvite.tsx
+++ b/packages/react/src/components/presentation/auth/AcceptInvite/AcceptInvite.tsx
@@ -3,6 +3,7 @@
 
 import {ThunderIDAPIError} from '@thunderid/browser';
 import {FC, ReactElement, ReactNode, useMemo} from 'react';
+import useTranslation from '../../../../hooks/useTranslation';
 import BaseAcceptInvite, {BaseAcceptInviteRenderProps, AcceptInviteFlowResponse} from './BaseAcceptInvite';
 
 /**
@@ -153,6 +154,7 @@ const AcceptInvite: FC<AcceptInviteProps> = ({
   showSubtitle = true,
 }: AcceptInviteProps): ReactElement => {
   // Extract from URL if not provided as props
+  const {currentLanguage} = useTranslation();
   const {executionId: urlExecutionId, inviteToken: urlInviteToken} = useMemo(() => getUrlParams(), []);
 
   const executionId: any = executionIdProp || urlExecutionId;
@@ -179,6 +181,7 @@ const AcceptInvite: FC<AcceptInviteProps> = ({
       body: JSON.stringify({
         ...payload,
         verbose: true,
+        locale: currentLanguage,
       }),
       headers: {
         Accept: 'application/json',
diff --git a/packages/react/src/components/presentation/auth/Recovery/Recovery.tsx b/packages/react/src/components/presentation/auth/Recovery/Recovery.tsx
index 82054100..1e794e6e 100644
--- a/packages/react/src/components/presentation/auth/Recovery/Recovery.tsx
+++ b/packages/react/src/components/presentation/auth/Recovery/Recovery.tsx
@@ -5,6 +5,7 @@ import {EmbeddedRecoveryFlowRequest, EmbeddedRecoveryFlowResponse, EmbeddedFlowT
 import {FC, PropsWithChildren, ReactElement, useCallback} from 'react';
 import BaseRecovery, {BaseRecoveryProps} from './BaseRecovery';
 import useThunderID from '../../../../contexts/ThunderID/useThunderID';
+import useTranslation from '../../../../hooks/useTranslation';
 
 export type RecoveryProps = PropsWithChildren<
   BaseRecoveryProps & {
@@ -54,6 +55,7 @@ const Recovery: FC<RecoveryProps> = ({
   ...rest
 }: RecoveryProps): ReactElement => {
   const {recover, isInitialized, applicationId} = useThunderID();
+  const {currentLanguage} = useTranslation();
 
   const handleInitialize: (payload?: EmbeddedRecoveryFlowRequest) => Promise<EmbeddedRecoveryFlowResponse> =
     useCallback(
@@ -71,7 +73,7 @@ const Recovery: FC<RecoveryProps> = ({
               executionId,
               inputs: {[tokenUrlParam]: tokenValue},
             };
-            return (await recover(resumePayload)) as EmbeddedRecoveryFlowResponse;
+            return (await recover({...resumePayload, locale: currentLanguage})) as EmbeddedRecoveryFlowResponse;
           }
         }
 
@@ -80,15 +82,15 @@ const Recovery: FC<RecoveryProps> = ({
           ...(effectiveApplicationId && {applicationId: effectiveApplicationId}),
         };
 
-        return (await recover(initialPayload)) as EmbeddedRecoveryFlowResponse;
+        return (await recover({...initialPayload, locale: currentLanguage})) as EmbeddedRecoveryFlowResponse;
       },
-      [applicationId, tokenUrlParam, recover],
+      [applicationId, tokenUrlParam, recover, currentLanguage],
     );
 
   const handleOnSubmit: (payload: EmbeddedRecoveryFlowRequest) => Promise<EmbeddedRecoveryFlowResponse> = useCallback(
     async (payload: EmbeddedRecoveryFlowRequest): Promise<EmbeddedRecoveryFlowResponse> =>
-      (await recover(payload)) as EmbeddedRecoveryFlowResponse,
-    [recover],
+      (await recover({...payload, locale: currentLanguage})) as EmbeddedRecoveryFlowResponse,
+    [recover, currentLanguage],
   );
 
   const handleComplete: (response: EmbeddedRecoveryFlowResponse) => void = useCallback(
diff --git a/packages/react/src/components/presentation/auth/SignIn/SignIn.tsx b/packages/react/src/components/presentation/auth/SignIn/SignIn.tsx
index 566ea98d..ab678eb4 100644
--- a/packages/react/src/components/presentation/auth/SignIn/SignIn.tsx
+++ b/packages/react/src/components/presentation/auth/SignIn/SignIn.tsx
@@ -250,7 +250,7 @@ const SignIn: FC<SignInProps> = ({
 }: SignInProps): ReactElement => {
   const {applicationId, afterSignInUrl, signIn, isInitialized, isLoading, meta, getStorageManager, scopes, vendor} =
     useThunderID();
-  const {t} = useTranslation(preferences?.i18n);
+  const {t, currentLanguage} = useTranslation(preferences?.i18n);
 
   // State management for the flow
   const [components, setComponents] = useState<EmbeddedFlowComponent[]>([]);
@@ -587,6 +587,7 @@ const SignIn: FC<SignInProps> = ({
           response = (await signIn({
             executionId: resumeExecutionId,
             ...(challengeTokenRef.current ? {challengeToken: challengeTokenRef.current} : {}),
+            locale: currentLanguage,
           })) as EmbeddedSignInFlowResponse;
         } catch (resumeError) {
           // Only treat a stale/expired session (HTTP 400 from the server, meaning the stored
@@ -620,6 +621,7 @@ const SignIn: FC<SignInProps> = ({
               applicationId: effectiveApplicationId,
               flowType: EmbeddedFlowType.Authentication,
               ...(scopes && {scopes}),
+              locale: currentLanguage,
             })) as EmbeddedSignInFlowResponse;
           } else {
             throw resumeError;
@@ -630,6 +632,7 @@ const SignIn: FC<SignInProps> = ({
           applicationId: effectiveApplicationId,
           flowType: EmbeddedFlowType.Authentication,
           ...(scopes && {scopes}),
+          locale: currentLanguage,
         })) as EmbeddedSignInFlowResponse;
       }
 
@@ -825,6 +828,7 @@ const SignIn: FC<SignInProps> = ({
         ...payload,
         inputs: processedInputs,
         ...(challengeTokenRef.current ? {challengeToken: challengeTokenRef.current} : {}),
+        locale: currentLanguage,
       })) as EmbeddedSignInFlowResponse;
 
       if (await handleRedirection(response)) {
diff --git a/packages/react/src/components/presentation/auth/SignUp/SignUp.tsx b/packages/react/src/components/presentation/auth/SignUp/SignUp.tsx
index 89a24243..fef47505 100644
--- a/packages/react/src/components/presentation/auth/SignUp/SignUp.tsx
+++ b/packages/react/src/components/presentation/auth/SignUp/SignUp.tsx
@@ -11,6 +11,7 @@ import {FC, ReactElement, ReactNode} from 'react';
 // eslint-disable-next-line import/no-named-as-default
 import BaseSignUp, {BaseSignUpProps, BaseSignUpRenderProps} from './BaseSignUp';
 import useThunderID from '../../../../contexts/ThunderID/useThunderID';
+import useTranslation from '../../../../hooks/useTranslation';
 
 /**
  * Render props function parameters (re-exported from BaseSignUp for convenience)
@@ -42,6 +43,7 @@ const SignUp: FC<SignUpProps> = ({
   ...rest
 }: SignUpProps): ReactElement => {
   const {signUp, isInitialized, applicationId, scopes} = useThunderID();
+  const {currentLanguage} = useTranslation();
 
   /**
    * Initialize the sign-up flow.
@@ -72,14 +74,14 @@ const SignUp: FC<SignUpProps> = ({
       initialPayload = payload;
     }
 
-    return (await signUp(initialPayload)) as EmbeddedSignUpFlowResponse;
+    return (await signUp({...initialPayload, locale: currentLanguage})) as EmbeddedSignUpFlowResponse;
   };
 
   /**
    * Handle sign-up steps.
    */
   const handleOnSubmit = async (payload: EmbeddedSignUpFlowRequest): Promise<EmbeddedSignUpFlowResponse> =>
-    (await signUp(payload)) as EmbeddedSignUpFlowResponse;
+    (await signUp({...payload, locale: currentLanguage})) as EmbeddedSignUpFlowResponse;
 
   /**
    * Handle successful sign-up and redirect.
diff --git a/packages/react/src/components/presentation/auth/SignUp/__tests__/SignUp.test.tsx b/packages/react/src/components/presentation/auth/SignUp/__tests__/SignUp.test.tsx
index 19cbbeab..6dc0303f 100644
--- a/packages/react/src/components/presentation/auth/SignUp/__tests__/SignUp.test.tsx
+++ b/packages/react/src/components/presentation/auth/SignUp/__tests__/SignUp.test.tsx
@@ -62,6 +62,31 @@ describe('SignUp', () => {
     window.history.replaceState({}, '', '/signup');
   });
 
+  it('attaches the current UI language as locale on the initiation request', async () => {
+    mockSignUp.mockResolvedValueOnce({executionId: 'exec-1', flowStatus: 'INCOMPLETE'});
+
+    renderSignUp();
+
+    await waitFor(() => {
+      expect(mockSignUp).toHaveBeenCalledTimes(1);
+    });
+    expect(mockSignUp).toHaveBeenCalledWith(expect.objectContaining({locale: expect.any(String)}));
+  });
+
+  it('attaches the current UI language as locale on a step submission', async () => {
+    mockSignUp.mockResolvedValueOnce({executionId: 'exec-1', flowStatus: 'INCOMPLETE'});
+    const captured = renderSignUp();
+    await waitFor(() => {
+      expect(mockSignUp).toHaveBeenCalledTimes(1);
+    });
+
+    mockSignUp.mockResolvedValueOnce({executionId: 'exec-1', flowStatus: 'INCOMPLETE'});
+    await act(async () => {
+      await captured.submit?.({id: 'continue'}, {username: 'user'}, true);
+    });
+    expect(mockSignUp).toHaveBeenLastCalledWith(expect.objectContaining({locale: expect.any(String)}));
+  });
+
   it('redirects to afterSignUpUrl when the completion carries an assertion', async () => {
     // A URL differing from the current one only by its fragment is a same-document navigation:
     // location updates without the page reloading, so the redirect can be observed from inside the