Add Multi-language Support for signInWithWebUI in amplify_flutter

[SugimotoRyunosuke]

Feature Request: Add Multi-language Support for signInWithWebUI in amplify_flutter

Summary

Currently, the signInWithWebUI() function in amplify_flutter does not support passing language parameters (e.g., lang=ja) for internationalization of the Cognito Managed Login UI.

Current Behavior

• Using amplify_flutter: ^2.6.5 with Flutter 3.32.5
• The signInWithWebUI() function works for authentication but displays only in English
• No way to pass language parameters to localize the managed login screen
• No workarounds currently available according to AWS Support

Expected Behavior

• Ability to pass language parameters (e.g., lang=ja) to signInWithWebUI()
• Support for Japanese and other languages in the Cognito Managed Login UI
• Consistent with AWS Cognito's built-in internationalization capabilities

Use Case

• Developing a Flutter application for Japanese users
• Product launch planned for February 2026
• Managed login is preferred over custom implementation for security and maintenance benefits
• Japanese localization is essential for user experience

Proposed Solution

Add language parameter support to signInWithWebUI() function, such as:

await Amplify.Auth.signInWithWebUI(
  provider: AuthProvider.cognito,
  options: const SignInWithWebUIOptions(
    language: 'ja', // or locale parameter
  ),
);

Additional Context

• AWS Support confirmed this feature is not currently available (Case reference available)
• AWS Support suggested creating this GitHub issue to potentially expedite development
• This affects developers building applications for non-English speaking markets
• Alternative custom implementation increases development complexity and maintenance overhead

Priority

High - This feature is blocking the use of Cognito Managed Login for international applications

Environment

• Flutter: 3.32.5
• amplify_flutter: ^2.6.5
• Platform: iOS/Android
• Region: Asia Pacific (Japan)

---

[harsh62]

@SugimotoRyunosuke Thanks for raising the feature request. Our team will look into the issue and check if we are able to prioritize it.