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For example how to deal with unsupported languages.
You can add multiple languages and Google Assistant (or other platforms) will tell Dialogflow what language to use.
Read more on this topic here,
Languages | Dialogflow Documentation | Google Cloud[
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Main target is to get audio or text input from end-user, automatically detect langauge which he speaks (as an option: translate it by third-party service) and decide which bot service supports that language then send request to that bot service for further processing.
This kind of a requirement would better use the Google Cloud Platform (if you want to stick to Google Cloud, otherwise Microsoft Azure, Alibaba Cloud, AWS, etc. they all offer a similar product) and perform these tasks separately before you process the message.
You can use speech transcription service to transcribe the audio and then send that audio to Dialogflow to detect the intent of the users.
Note that most of the platforms, such as Google Assistant, will automatically transcribe the speech for you.