I mean strictly speaking by using a language other than the native one of the court you were conspiring with your mother.

Conspiring is just planning or acting jointly with another in secret in its most mundane form.

If your hiding your conversation by changing the language that is text book boring mundane conspiracy.

Even if you didn’t intend to hide it, you were in fact doing so. It’s entirely understandable for a court to demand you speak the language of the court or have a translator present to ensure everyone understands.