Several methods exist to detect the language of the user intent.
Redirect the user to the proper site (recommended by Inbenta)
After you detect that the user intent uses one of your other instance languages, you can show a message that offers to redirect them to the site that has a chatbot in this language. This is particularly recommended for integrations that use the SDK languageDetector method of the JS client.
The best possible method to present this message to end users is by creating one or more specific intents in the Chatbot KB.
Create specific intents in the Chatbot Knowledge Base (recommended by Inbenta)
You can either create one single intent or dialog with specific message(s) in the Knowledge Base for all languages, or one per language. To call this intent, you have two options:
You can use either one of these methods to easily change the message to your users without changing the code deployed in production. The adapter documented here includes a full example of this option: It uses a direct call to a specific dialog to handle the response to the user, based on the detected language results.
The image below shows the dialog that the adapter uses:
These are Inbenta's suggestions for the best way to handle user intents in unexpected languages, but other solutions are possible.