From what is documented today, I would treat this as not currently supported as a Gboard-style automatic language switch.

LeanType has multilingual typing and multiple dictionaries/layouts, but the active suggestion/autocorrect path is still tied to the selected input language/subtype. The AI features in the standard build can proofread or translate text, but that is different from automatically changing the live correction dictionary after the first word or two. It also would not fit the offline/offline-lite builds, where network AI features are intentionally unavailable.

So the current practical workflow is still to switch languages manually, or use the direct IME/language switching features if you want faster switching. A real implementation would probably need a separate feature: detect the language from the current composing text, switch the active dictionary/subtype, and be careful not to break mixed-language typing or privacy/offline behavior.

If this is the behavior you need, I would open it as a feature request rather than looking for a hidden setting in v3.9.1.

If this answers the question, please mark it as the answer so other multilingual users can find the current limitation.