If your agent doesn't speak the language you want it to, check if:
The language is put to the language you want
The instructions are written in the language you want
The greeting is in the language you want
For advanced agents, the steps and when to go to next steps are written in the language you want.
And if you really want to be sure, you can add to the instructions "Always speak in [language]".
For multilingual agents this does not work of course, so then you can just write the instructions in the language you prefer but add this to the instructions: "You can speak English, Spanish, French, German, Hindi, Russian, Portuguese, Japanese, Italian and Dutch".
The language it will start speaking in is dependent on the greeting message. If that's English, the agent will start speaking English.