Amazon Lex language integration failures in my Amazon Connect instance.

It sounds like you're experiencing a language integration issue between Amazon Lex and Amazon Connect. This is a common problem that can occur even when the bot works correctly on its own. Here are a few things to check and try:

1. Language attribute: Make sure you've set the language attribute correctly in your Connect flow. Even if you've selected a French voice in the 'set voice' block, you need to explicitly set the language attribute to 'fr-FR' in the flow component. This is a crucial step that's often overlooked.

2. Bot configuration: Verify that your Lex bot is properly configured to support French (fr-FR). Ensure that you have intents, sample utterances, and responses set up for the French language in your Lex bot.

3. Alias and version: Double-check that the alias you're using in Connect ('my alias') points to a version of your bot that includes French language support.

4. IAM permissions: Ensure that the IAM role associated with your Connect instance has the necessary permissions to invoke the Lex V2 bot, specifically for the French language version.

5. Region consistency: Although you mentioned that Lex and Connect are in the same region, it's worth double-checking this, as region mismatches can cause similar issues.

6. Resource quotas: Verify that you haven't exceeded any resource quotas when associating the bot with Connect, particularly for multiple language support.

7. CloudWatch logs: Examine the CloudWatch logs for both Connect and Lex to look for more detailed error messages that might provide additional clues about the cause of the 'ResourceNotFoundException' error.

If you've verified all these points and are still experiencing issues, you may need to start with a simplified setup. Create a basic flow and bot configuration with minimal French language support, then gradually add complexity to identify where the issue occurs. If the problem persists, it may be necessary to contact AWS support for more in-depth troubleshooting, as they can access more detailed logs and configuration information.

Sources
Struggling to get Lex v2 bot to work with Connect | AWS re:Post
Lex v2 with Connect not working | AWS re:Post