"They would like to speak in French."
Translation:Elles voudraient parler en français.
Yes you were.
Je parle français = Je parle en français
But be aware that the 2nd option can only be applied to a specific situation in which you need to speak French. E.g. "Avec Pierre, je parle (en) français". So if someone asks "What language do they speak in Wallonia?" you'd answer "Ils parlent (le) français". You can't use "en" but can use "le" in this context.
Hope it helps